It's hard to review this headphone as it's the most I have ever spent on a headphone. For starters, I don't really care for the HD800, I do have the SA5000 which I consider much more technical. So of all three which would be the most musical, while delivering an accurate sound? The Beyerdynamic T1 of course!

The sound signature is inoffensive, which is how I would describe it, it's more laid back than the Q701 and DT880. But the treble is never boring like the HD600/650. It can sparkle when you need it and it's not very fatiguing. The downside of this is that it could get a bit boring for some, especially if you're accustomed to a hot signature.  The bass is perfect in texture, slam, weight... simply everything. This is the best bass I have ever heard, it's neutral bass and not for bass-heads, but a 30hz file rattles my head. And the midrange is upfront and clear. A perfect harmony, not achieved by any other headphone I have ever heard (LCD2, HD600, HD800, SA5000, Q701, DT880, etc).

It may seem I am bashing others, but the T1 can do no wrong. It's just very balanced. It's not really an end-game, but an amazing reference. I do think I prefer the Q701 for pure enjoyment though (I'll get on that later).

The soundstage is big and airy, but not like the HD800/Q701. If anything it's not average, but somewhat like the good ol HD555, big and spacious, but not overly big and spacious. It does intimacy much better than the HD800/Q701.

Everything sounds great on these, classical, rock, metal, techno, jazz. It really has the punch for rock/metal despite what many says. The transients seem quick and clear, but not like the SA5000 or the HD800.

Detail... It's still mid-fi class when it comes to detail, it should be compared to the Q701, DT880 and HD600. It just can't resolve as much as the HD800 and SA5000. Whether this is better for you, I can't decide. The good thing is that you can focus on the music, instead of the hidden details. It is very detailed though, don't expect a very high level of micro-detail retrieval, if you're upgrading from the DT880.

It's certainly not fatiguing, and the comfort is excellent. For long listening sessions, it works very well. I forgot to mention, my serial number is in the 5000's range. I don't know if many major revisions were made to them.

Price/performance? Not very good, it's slightly better than the DT880, imagine a much more refined DT880. That's the T1.

Back to enjoyment, here we have a headphone that does nothing wrong, and does everything very well. But it does not manage to be spectacular at something, not at detail retreival, not at soundstage, etc.
I find myself using the Q701 for classical music, and I might use the T1 for classical music that's bass heavy (organs, drums, etc).

I need more listening hours on these to truly decide my opinion. They have roughly 75 hours of listening time. I sold my D7000's to purchase these and I think they are much much better than the D7000's.

These headphones are enjoying a period of popularity. I suspect that it is due to them being discontinued. But be warned... they are not for everyone.

These headphones are by far the most revealing headphone I have listened to. It's unnaturally revealing, and while far from neutral (slight V shaped, but with rolled off bass), the midrange isn't recessed, it's very clear but the mid bass and treble is quite pronounced.

Speed, or perceived speed is the best I have heard in any full sized headphone and I've heard the HD800, T1, K701. They just can't compete.

The detail is amazing, you will hear things you have never heard, even if you listen to revealing cans. To understand the detail, you really need to spend time with the SA5000. When compared with the HD800, it is quite a bit more detailed, but it would not seem so due the the HD800's superior instrument separation, which makes picking the details easier.

The SA5000 is what I would call the ultimate technical headphone, and it may not be very musical, some like to pair it with tube amps to achieve musicality.

The soundstage is not wide, but not small either, I would rate it average for those of you concerned with it.

Onto the cons, the brightness of the treble can be sibilant at times, or just plain fatiguing. The cable is microphonic (the first time I have experienced this on a full sized headphone). And I have read reports of many having build quality issues. I have never had problems with mine. There is also very little sub-bass. Mid-bass is very fine though.

On amping, they are easy to drive and the budget Fiio amps work well, but they deserve something cleaner such as the O2.

They work well for fast musical genres that isn't bass heavy, namely metal and techno.
 
Oh and they come with the best headphone stand ever.