These are a great update to the ZS5, but aren't necessarily worth it if you already own the 5. The sound is nearly identical, including the harsh treble and boost in the presence range. If you broke your plastic ZS5, get these- they're metal and won't break the same way. They don't address any shortcomings except build quality.

Where the 5 was an amazing value, the 6 is that much more of a deal for being metal. That said, expect the same issues with SQ. Non-existent to poor imaging and brutally harsh treble zing, but for $35 shipped, zero complaints.

*UPDATE*

I ordered the silver cable ($19) and a set of Ostry tips ($12).

I can't tell if the cable makes a difference, but Ostry red tips tamed the treble. Keep in mind that these are no longer $35 headphones- they're closer to $70 after shipping. Was it worth it? Emphatic YES. You'd be fine just ordering Ostry reds ($8) instead of the variety pack. The new cable is way less microphonic, but is also missing the microphone and control button. I'm happy with it, but it's not necessary for SQ unless you lost the factory cable QC lottery.

If you want to give these headphones a shot, order the Ostry tips at the same time.

These are still a 4.0 out of the box and it's not really fair to hand them a 5 for non-factory upgrades which double the price. SQ is definitely a 5 now, but they should come sounding like this straight from the factory. Even with the upgrades they're still a tremendous value. Highly recommend!

I didn't know what to expect from these- I paid $18 each for two pairs, with the idea that if they're good I'd give them as a gift. I'm kind of obsessed with sound quality, and needed to hear them before handing them off. Got a sweet discount for buying a second pair, so problem solved.

While the packaging is pretty nice, these don't come with many accessories. There's no pouch or case, and only three sets of tips, small, medium, and large. The cable is rubbery but has nice strain relief, a passable splitter, some memory wire on the ends, and comes terminated in a plastic right angle jack. I'm impressed that the cable is removable- I have a balanced cable, but won't be able to test these balanced until my new amp shows up.

The build quality is not stellar, there are sharp edges, panel gaps, molding marks, and even some flashing still attached in places. Not too concerned with that though, some sandpaper and glue and everything will be fine. I mean... $18, right?

These are not comfortable in any way, shape, or form. The sharp edges and size, combined with the need to keep them deeply inserted for SQ make them painful to wear. I'll fine tune the fit and kill those edges when I clean up the plastic, but my hopes aren't that high.

I tested these in a pretty unfair way- I have some Fostex x Massdrop TH-X00 Purple Hearts with Dekoni pads and a set of three way DRMEARZ customs. I used a Fiio X5 Mk. I, both from the headphone out directly, and through the coax out into a Bifrost Uber/Asgard 2 stack.

I used two tracks- Piles from the Re-Volts EP in VBR MP3 at 320, and Kendrick Lamar's LOYALTY in WAV.

The first thing that struck me was the sheer amount of bass. I've never heard anything like it from headphones. There's just so much volume and extension. All the stuff about cheap build quality and discomfort just went right out the window when the music drops out in LOYALTY and it's just the baseline and Kendrick- I've heard subs that don't hit as hard as these IEMs. Totally nuts! The ZS5 just walks away from both sets of $500 headphones. Didn't expect that at all. Blew me away!

The mids aren't anything special, but I didn't catch any flaws either. The bass doesn't intrude too much, so while there's a slight mid bass hump, it's pleasant to listen to. The more expensive options just do it better, but who is really surprised by that?

The highs are a little veiled, but that's a good thing because they're harsh. This is where the ZS5 really falls down in this comparison. They're noticeably harsh with some extra zing in the presence range and have a cheap, non-hifi edge.

So... are they worth $18? YES

They're fantastic for the price, and would be worth it even in the $50-75 range as is. What I'd like to see is a version with a better enclosure, a storage case, and *** make them comfortable to wear for more than an hour at a shot. They legitimately hurt to wear, but the sound is so good that it's worth it in short bursts. KZ, if you read this, if you sort out the comfort issue and tame the treble a bit, all you'd have to do is add a case and these would be legitimately worth $150.

This is a quickie review... not the standard head-fi thing.
 
I've been listening to the Mk III T50RP for a week now and I've been able to compare them directly to Sennheiser's Amperior and HD580, Fidelio X2, Grado SR125, Meelec A151, and DreamEarz VOX3 CIEMs. Source is a Fiio X5 HO or LO feeding a Schiit Asgard 2. 
 
The X2 has been my reference for a year of listening now, and the first time I picked up the T50 I knew I liked it better. Crazy!
 
I have a thing for fast, neutral headphones. My previous favorite was the Ety HF2, but I no longer have them. I switched to Meelec A151, which are great, but are now just for listening off the phone. Those single driver BA IEMs just have magic in them... the T50 is the closest dynamic headphone I've heard to that single BA sound. There's just so much more bass. Compared to the X2, they're faster, lighter, and have more slam in the bass. Going from the famously veiled (and slow) Senns to the X2, I noticed a slight change in smoothness- the X2 picks up some clarity, speed, and bass extension, which are all great, but wind up being harsher than the Senns. I liked them because they had a similar sound sig to the HD580, just better all around. The T50 has a radically different sound sig, but it's just so fast and clear that I fell in love immediately. The bass extension is like nothing I've heard from headphones except maybe in the VOX3s and those have a whole host of problems. Even the insane bass of the Amperior doesn't come close. These cans are fantastic.
 
What I'm not sold on is the mids. I like my midrange warm and sweet, but these are a shade too recessed to be called flat, and it's definitely dry. The treble leans closer to the "boom and tizz" side of the spectrum than the hifi slant. From what I can tell, there are mods to "fix" these little issues even if they're designed for the MK II. I can't wait! 
 
For the price, there's nothing like the T50RP... now to start my mods

I got my X2 Christmas day and now have 5-6 hours listening and 20+ burning in. I use a Headroom Total Bithead and iTunes with ALAC Redbook on a 2011 MacBook Pro. My other full size headphones are a pair of modded Sennheiser HD 580s with HD600 grills and a HD650 cable and some Grado SR125s with stock cable, new stock ear pads, and a lambskin headband. Also own (but not used for this review) TTVJ Millet Hybrid Portable, Senn Amperior, Ety HF2, UE TF.10 Pro (981/1000), and MEElec A151P. My main system is DefTech BP-6Bs, Arcam A80, Schiit Bifrost Uber USB, fully upgraded Pro-Ject Debut III, and Audioquest cables. I listen to hip-hop, pop, and rock. Most of my headphone listening is with my computer. The tracks I used for evaluation are Daft Punk's Give Life Back to Music from Random Access Memories, Dinosaur Jr.'s Pierce the Morning Rain from I Bet on Sky, and Kendrick Lamar's Money Trees from Good Kid M.A.A.D. City. 

First off, I love the X2. They have a great sense of space, wonderful mids, and deep bass. I've been most impressed with their ability to preserve dynamics in recordings, but bass reproduction is a close second.
 
Comfort:  HD580>X2>SR125
 
The Senns are slightly more comfortable due to the large ovals and foam vs the pure circle and elastic of the X2, but that's splitting hairs. Both sets are insanely comfortable and wear well for several hours. The Grados are very light, but the foam isn't soft and sits on the ear rather than around your head. The Grado headband has no clamping force, so they're not very secure either. Not great for long listening sessions or when you have to move around. The Senns and X2 both stay put. 
 
SQ: All three are equal, but for different reasons. The HD580 is neutral and smooth with good detail retrieval, the SR125 is quick and airy, and the X2 is a great all-rounder with decent detail retrieval, fantastic dynamics, better bass than both, and a hint of warmth. 
 
The X2 is slightly more mid-forward than neutral which gives them a warm feel, but it's only noticeable in A/B tests with the Senns. The Senns are crazy neutral and don't dig as deep as the X2 or have the treble extension of the X2. The Senns aren't as dynamic as the X2. The Grados aren't either. The Grados are faster than the X2 and have a boost in the presence range. The Grados have better attack and speed- the X2 and Senns both feel slow in comparison, like the difference between BA and dynamic IEMs. The Grados isolate notes much better than the X2 or Senns, but that clarity comes from their speed and a boost in the presence range. Definitely not neutral, but lacks the bass impact to be truly V shaped. 

My immediate impression is that the HD580 and X2 have nearly identical neutral signatures, with the X2 having more solid bass, a slightly bumped up midrange, and a better sense of space and dynamics. The X2 is slightly less smooth than the HD580, but the detail is still all there. The differences are slight except for bass response and dynamic performance. The extra bass extension is immediately noticeable, dynamics take a few listens to hear the differences. Against the Grados, the X2 sounds fuller but slightly less realistic, especially with cymbals and snares. 

Overall, the X2 is fantastic. It's comfortable and sounds great stacked against a fun headphone and a true (if aging) reference headphone. I only need one set of full size cans, and I'd be happy to keep the X2. I'll be selling the SR125 and HD580 because I've found my keeper set of cans. Check the FS section for the listings. 
 
Update: Tried the X2 with Laptop --> Bifrost--> Millet Hybrid. The X2 scales really well. Sounds great with both tubes and solid state. My millet swings 8+ volts, which probably explains how the bass solidified vs. the Total Bithead. Imaging and separation improved as well, but that could be the DAC. I'm just happy I can hear the differences- I don't know why some people are calling the X2 mid-fi. It's clearly hi-fi to my ears.

Edit: Will report back here with Fidelio X2/Fiio X5 impressions when the X5 arrives. My X5 hasn't shipped yet, but I plan on using it to replace the Total Bithead as a USB amp/DAC with the added bonus of playing off the grid too. 

UPDATE!

I don't want to get lost in audiophile nonsense language here. The X5 HO is amazing- it's clarity and control are the best I've heard. Not for a portable. For anything. This is a dream combo. 

The story doesn't end there... I stacked the X5 with my PMHA. The X5 running a TTVJ Portable Hybrid with the X2s is insane.

The PMHA is sidegrade or even downgrade in some areas compared to the HO of the X5, for example, bass is noticeably flabby in comparison. I don't even care. There's way more of it. The realism that the tubes give is worth it. Call it midrange bloom, weight, bass slam, whatever. Doesn't matter what you call it, you just have to hear the difference on a suitable recording. I'm using an ALAC rip of Opeth's "Ghost of Perdition" from Ghost Reveries. It's perfect for this combo. It's brutal authority with glowing mids and effortless treble. This combo is worth its weight in gold. For the first time ever, I'm not sorry about my wallet.