I've put a fair amount of time on these T5Ps (Rev 1, I believe. They are an early model), so I'm confident enough to post some impressions.
The most noticeable thing right off the bat was the ridiculous soundstage. It has a huge presence, but nothing ever feels distant. It's like sitting with your jaw on the stage at a concert. Just absolutely in your face, huge, and front-projected.
Mids are superb. Very detailed, and the overall sound is edging more toward forward, which gives them a huge amount of energy. I think the CD900STs might have more detail, but they're a little more laid-back sounding. It's a different enough flavor for me to justifty keeping both.
Moving into the highs, again, it's a little more forward. Never ear shattering, just slightly elevated. This, too, gives a greater sense of energy. Acoustic guitars have a nice pluck to them, pianos have lovely sparkle, and harmonicas have nice body to them without being ear-shredding. (This was one thing I was afraid of, since Bob Dylan is like 40% of my music)
Bass is where things are kind of interesting to me. They have less than I expected. It extends low, and it's reasonably punchy (certainly punchy enough for me), but I was sorta expecting something close in quantity to the 770 Pros. They come in a bit shy of those, though. The plus side, is they're VERY quick, and again, extremely detailed. There are some muddled tracks I've heard even on the CD900STs where it's hard to follow a bass line if there's heavy bass drums, but here it's perfectly clean.
So overall, a very well-rounded pair of headphones, leaning toward quick and forward. Basically, they seem built from the ground up for rock, which completely suits my tastes.
Oh, and isolation is even better than the 770 Pros! Comfort isn't QUITE as nice, since the cups aren't as deep, and they don't use the velour pads, but still, very comfortable.
AND, they're absurdly easy to drive. I run them out of my phone from time to time, and they are marvelously well controlled, with plenty of headroom to raise the volume. There's some serious sorcery going on at Beyerdynamic's R&D labs to make a headphone that sounds THIS good without amping. If anyone is looking to jump into a proper high end setup, definitely read into these. You can save yourself hundreds of dollars that you would have spend on a high-end amp for any other headphone in it's price range.
In direct comparison to the 770 Pros, these have slightly less bass, but overall better controlled. Mids and highs are more forward, and soundstage is much bigger. Better isolation, but slightly worse comfort. Even easier to drive than the 770s. (!)