Me and my non-head-fi friend spent an evening listening to the HD800.
 
We've both spent a lot of time with the HD650, but I had the SR-007 and HD800 available that we took turns listening to.
The HD800 was powered by a Headamp GS-1 and Assemblage 3.1.
 
Here are some notes:
The HD650 has a 2-blob soundstage, which the HD800 don't have, and that makes the soundstage larger, and can be a little bewildering to listen to if you're trying to pick out detail and passages.
 
Stax 007 provides details with greater ease. But it's really apples and oranges, the Stax and HD800 couldn't be more different.
Since the HD800 is so critical and detailed it helps to take a step back and try to hear the music as it comes, without being hyper vigilant as you listen to it.
This involves a new methodology of listening where you consciously focus on absorbing the information.
This is why me and my friend decided it was best to use random music choices to keep us interested.
 
In general, I agree with what someone else said about having other gear that you can enjoy before you acquire the HD800 for casual music listening.
Because it's nice to take breaks from focusing on absorbing the HD800's musical information.
If you don't have any other headphones, or simply can't enjoy music without hyper vigilance the HD800 likely isn't for you.
In this way it follows logically that it would be helpful to have some experience with other headphones in the hobby before approaching the HD800.
Compared to the 007, the HD800's detail and resolution power don't seem to match as well with a soundstage so big. There's so much information, but it isn't always possible to process it all meaningfully.
This is something I commonly experience on other dynamic headphones. My friend has only really heard an HD650 and some low range AKG's so he couldn't comment.
 
But even he was able to respect the HD800 for what it can do in analytical applications, where it's imaging can really shine, music production, movies, and turning up loudly to simulate being at a music festival.
 
I've decided that the HD800, if personified, would be symbolized by a person with a sort of refined sensibility, who subscribes to moral standards and etiquette.
They like to be 'understood' and not just 'listened to'. They like to be 'felt' as well as 'heard'.
 
Is this some sort of poetic expression for the meaning of life?
And the 8 in 800 an upright expression of infinity. Are the two 0's side-by-side really a symbol of infinity.
"HD8∞." It's an interesting proposition.
If I had an infinite amount of disposable money I wouldn't be surprised if I could discover new ways of appreciating the HD800 with IC upgrades, new cables, tube amps, etc.
But I wouldn't be surprised if really all I was doing just that, discovering new ways of appreciating it.
There's so much information to absorb. So much to experience and take in. d(*-*)b
That said, if I turn up the volume to a higher-than-usual level it sounds like my favorite electronic music festival, so a little drum and bass now and then doesn't seem to offend the HD800's puritanical pursuit of truth and justice in the universe.

EDIT: These headphones with a Dynahi is jaw-dropping. A little bit fatiguing, but it gives me most awe of any headphone setup I've heard. This shows imo that the headphones can scale well with equipment upgrades.

This is my reference for what a portable closed headphone should be.
It's neutral and sounds good on all genres.
It's not a giant killer like the DT531, but keep in mind it is closed and portable and has the most durable build quality of anything I've seen.
Also, it has the cheapest, most cost effective upgrade available for it, the upgrade to an HD650 cable.

In my opinion it is the second best deal to be had in headphones for a beginner to this hobby, second only to the Beyerdynamic DT531, if you can find it.
The Beyerdynamic DT531 has better high range sparkle and soundstage, but less midrange realism, depending on the genre you're listening to.
Anyways, neither are a perfect headphone, and calling either a giant killer is an exaggeration.
What I mean by that is for someone new to the hobby, the Beyerdynamic DT531 and Senn HD25-1 are just an amazing deal in terms of price: performance ratio.
Yes they are bested by even mid-fi headphones in the $600 range. But considering the price you can find these for used, I'm not sure I'd recommend someone to 'upgrade'  and spend 500$+ an amp if they're happy with the sound.

Listening to John Coltrane - A Love Supreme I preferred the HD25-1 to my HD650 balanced system powered by a Beta 22/Lavry DA10.
The HD25-1 has a seriously sweet midrange for being as neutral as it is. Tonally accurate, yet easily heard in the music, almost 'highlighted'.
This mysterious midrange quality is it's greatest strength and the most stand-out part of the headphone followed immediately by it's tight, tuneful base.

Now for the negative part. Over time the closed in soundstage makes them less fun to listen to. It kind of builds on you over time and threatens to distract you completely from the listening experience.
But if you listen while doing other things, like snowboarding or skateboarding, biking, you don't notice this.

That's all I really have to say for negatives. For the right person, these are the ultimate portable and rugged, yet amazing sounding headphones.
If they redesigned the arc assembly so that the headphones folded outward when sitting on your neck, we'd have a new FOTM (flavour of the month) on head-fi.

I'm an Audio Technica fanboy, and I love their sound.

Compared to the AD2000 headphone with a Gilmore Lite and Bel Canto DAC3, I actually didn't miss it when I only had my ES7.
Just a nice headphone to have for fun listening. I used the 'phat pad' mod for both.
The ES7 and the AD2000 share a more similar sound signature than the other ATH cans.
 
The two extremes that the various Audio-Technica headphones seem to try to balance, 1) coloured mids and 2) balanced frequency response and PRaT.
The happy medium would be considered a 'fun' headphone.
Some of AudioTechnica's headphones can come across as tedious, depending on your personal taste of course. The W5000 and AD2000 come to mind. Technically proficient with detailed, balanced enough frequency response and PRaT but not as much about 'fun'.
Because of the trademark Audio-Technica sweet mids they are all all are indeed musical to a certain degree.
 
The W11jpn are the closest I've heard to bridging the gap between the two extremes of having sweet, musical mids while maintaining balanced frequency response/detail/PRaT.
The W3000anv is more about coloured mids and musicality than about balanced frequency response, so it would win the 'most musical' award.

Those particular rare headphone aside, the ES7 hit the 'fun' nail on the head. It has a balanced frequency response and the ATH musical midrange, what more could you ask for?
Neutral enough to be considered an all-rounder for most genres of music, sweet enough sounding to specialise in certain genres.

I owned these and the Senn HD25-1 concurrently and it was hard to decide which to reach for a lot of days.
The HD25-1 (with HD650 cable) is technically more advanced & more neutral, but the ES7 was a specialist with the genres I was listening to at the time.
These were psy-breaks, psy-trance, hip-hop.
I reached for the Senn HD25-1 more for jazz and downtempo.

For some reason I'm more sentimentally drawn to the ES7 than the HD25-1, maybe it's the mirror finish which indeed inspires pride in ownership.

The HD650's don't do anything really wrong.
They have a natural sound signature and slightly elevated midbass that is easy on the ear (non-fatiguing).
Most people will find that they appreciate their sound.
They are not the final word in transparency and speed, but they are advanced enough technically to allow them to be true to the source material, offering an excellent sonic window into the music.
This neutrality allows them to be used for reference purposes; as a studio headphone mixes done on them translate well to monitors.
Similarly, they sound great with all genres, and don't dominate any particular one.
 
For a beginner head-fi'er, HD650's paired with a <$500 SS amp and a cheap DAC might be the last headphone rig you ever need.
Inversely, the HD650 scales very well with upgrades, sounding best to these ears with a powerful tube amp.
However, If you don't like the sound of these phones on first (amped) listen, don't try change the sound signature with cable upgrades because the differences are relatively subtle.

The first thing I noticed with these headphones is clarity. -These are the only headphones that I have heard that can execute perfect imaging while generating a fully independent bass sound right in the middle of the soundstage.
This is the first time I have heard sound so holistically reproduced, and as an audio engineer I find this to be especially inspiring.
With other headphones, (particularly dynamics) the bass distorts the coherency of the sonic image and two sounds often compete for the same area of soundstage.
Comparatively, the 007 have uninhibited resolution and inner layering of detail which coalesces into a holographic audio image.
It's hard to imagine headphones being any better, and when it comes to amplification this is just the tip of the iceberg for me and my 717.
The best amps available are the Woo WES, Headamp KGSS & BHSE and the DIY KGSSHV.
Although the 717 is the budget amp for these headphones, this rig has changed the way I listen to music, and how I form criteria for gear I audition.
 
Before arriving at electrostats I built a balanced dynamic headphone system, owned the AKG K340 (hybrid of dynamic & electrostat), and the YH100 (orthodynamic).
So far the electrostatic technology that I have heard is the most advanced, although I am also watching out for new orthodynamic offerings as they become available.
At this point I have largely lost interest in dynamic headphones.

I have experienced a nicely sized 'comfort zone' with these speakers.
There is a slightly larger horizontal comfort zone than with my Mackie HR824's.
I was using these speakers in the studio but I wouldn't recommend them for reference monitors because the low mids sounded too attenuated to my ears.
An application where the MediaDesk will shine however is in a a den or a small room. These speakers are great for smaller spaces and low-level listening because at lower volumes the lower mid attenuation is much less noticeable. (re Fletcher-Munson curve)
 
The system also allows for an upgrade to a 5.1 set-up, with additional active amps being placed in the subwoofer casing.
Overall Blue Sky did a pretty nice job on these and after hearing them I am more interested in hearing Blue Sky's TOTL System One which is currently used in some big name movie production studios.