This little amp is a surprise. Alone it is actually not that bad with some HD650's. The sound is typical dark, liquid, and pretty nice to listen to if you're into that sort of tubey coloration.
 
Later, when you try to put this DAC into a revealing system, perhaps as a source for a revealing amplifier, the surprise is that the uDac 2 is actually quite terrible. And it has nothing to do with anything that has been ranted and raved about, forget the tech specs. The DAC section simply has a flat out horrid frequency response, it's boosting the upper mids in a way that makes revealing amplifiers sound ear piercingly bad...  This DAC could go well driving a tubey sounding tube amp, but anything even a little revealing is going to be terrible sounding.
 
Build quality is decent, it is metal and the connectors are gold plated. The first version (uDac v1) had problems with channel balance on the volume pot, that seems to be largely alleviated in the uDac 2. I do not have a channel imbalance issue with my uDac 2. My uDac (v1) on the other hand was almost unusable because the channel imbalance showed up at a relatively loud volume.
 
Once you add up everything it does, few amps can match it in features/price, and it really does sound good on its own. But if you plan on building a system and upgrading it and hope to use the DAC section of the uDac 2 with a better amp, just DON'T, save you and your ears the trouble.

These were my first foray into high end, high fidelity sound. They were a true revelation. These headphones were built for my ears, laid back, rich warm mids, deep but controlled bass, the de-facto ultimate headphones of their time.
 
Treble: Laid back, airy, great sound stage, contrary to popular belief, bigger is not always better with regards to sound staging, some things weren't recorded on a giant sound stage afterall. Liquid cool and never fatiguing, the treble was far from analytical but still revealing of source quality, a transparent headphone to be sure.
 
Mids: Wow. Rich palpable midrange to die for. This is what you dream of. Perfect vocals, anyone could hear the sound of a singing voice coming from these and be instantly transported. The mids on the HD 600's are capable of amazing things on the right music with the right amp. Still punchy and musical on almost all music types. My only complaint was with symphonic music, which always felt a little strained to my ears. In the end, I loved these most with the xcan v2, perfect, perfect vocals, any singing voice shone through on that setup like the voice of angels.
 
Lows: Rich enveloping bass cradles the music in a seamless landscape of liquid sound. Can be boomy on some tracks. Not the best headphones for rap or other excessively bass heavy music.
 
Very comfortable.
 
High build quality.
 
An excellent headphone which with the proper amplification (tubes) has a unique sonic signature, that if it works for you, is nearly impossible to beat at any price.

These headphones are kind of an enigma. They look like cheap crap, feel like cheap crap, and unless you amp them properly, sound like cheap crap too. With a good amp these phones WILL go deep with tight detailed bass. The enigma here is that yes, these CAN sound good, but you will need to spend $$$ on an amp. At that point you have to ask yourself, if you spent money on an amp, should you in turn spend money on headphones? Yeah, you probably should... If you can find a cheap amp that has some real current (it plugs into a wall not a USB port) then you should be good to go.
 
Treble: Dry, slightly fatiguing, detailed and airy with a good amp.
 
Mids: Slightly recessed mids, can deliver a little bit of punch and sparkle, vocals can sound really good with these. Sound stage collapses with more complex symphonic music.
 
Bass: Without amplification bass extension is poor and mid bass seems a tad tipped up. With amplification the bass extension is decent, musical down to about 30-40hz most likely.
 
Overall sound impressions: I used them occasionally but found that they really only sound good on certain music types, for symphonic music you need to check elsewhere, but these will rock your world with vocals and acoustic guitar. A tube amp that can handle 32 ohms does wonders for these cans midrange.
 
Moderately comfortable, lightweight, not built to last years and years, but they haven't broken yet and they frequently get moved around. Make sure you adjust the headband, it helps keep them in place on your head. Unlike some higher end senns, these do not clamp your head.

Sound is only a little better than average at best.
 
Treble: Analytical sounding and a little bright for my tastes, a little fatiguing, liquid though lacking absolute detail, some air, compressed sound stage.
 
Mids: On the thin side, but fairly detailed and liquid, lacks defining punch and rich tonality. Voices sound good but lack that palpable richness of say, HD 600's.
 
Bass: For headphones the size of sliced grapefruits these surprise with their less than ideal bass extension. The bass they do have is decent though, not boomy, well controlled but not particularly detailed.
 
Overall sound: Well, I gave them to my girlfriend and she likes them well enough and uses them at her desk job.
 
They're quite comfortable, perfect for long listening sessions.
 
They isolate pretty well.
 
They look ridiculous after you get over the wow, shiny blue metal phase.
 
After factoring in the superb build quality, high comfort, and decent looks if you like the color, these are not bad for $170. I'd hesitate to pay more though.

Great sound, typical dark Sennheiser tonal balance that I personally like. Sound for the money and construction is 5 stars. From an absolute sound quality falls somewhere short of that, but is quite good for the application/form factor.
 
Best for rock, beats, vocals, etc, simpler more intimate music types shine. Symphonies are the weak point.
 
Construction is solid, no worries with these, exercise, put them in the gym bag, glove box, whatever. 
 
Comfort is the one down fall, large ear buds may pain some people over time. They don't feel particularly secure on my head but haven't fallen off either.
 
Quirks, bud covers come off quite easily, haven't lost them yet though even after a year. I think they sound better without them anyway. In-line volume control is of negligible value, it does not go to zero... Maybe intended as a simple way to adjust impedance? I find some amps sound better with these if I set the in-line to full minus and turn the amp up to compensate, works wonders with some types of amplification. Dynamic take a hit but tonal balance is vastly improved (not an issue with your typical portable out)
 
Some sound isolation, depends on your ear shape I would suppose, the hum of a computer may be largely or partially drowned out with these on and music at moderate levels.
 
All in all solid headphones, worth the $$ for the application or just sheer worry free portability. They look like everything else so won't attract much attention either.