I tried the P7 at a Best Buy last night. Unfortunately, the headphone demonstration system at the Magnolia section does not allow one to use a separate source. I was ready with my iPod Video/FiiO E12, too. So, my opinions are heavily weighted by the setup they had, which had lots of things in the sound chain, including some sort of rotary volume control that was digital in nature (rotations led to step changes in the sound level, but only if you rotated the knob a half-turn or so, and definitely not proportional).
I compared the P7 to the P5 which was next door and easily accessible.
Comfortwise, the P7 is definitely a snug over-ear design for me. It is hard to tell whether it will be like the Momentum -- comfort depending on ear size. I was fine with it, and I felt no discomfort, but it did feel like my ears were being cradled, very much like the M100 with XL pads (which I own).
Soundwise, the P7 has a much brighter sound signature than the P5. The P5 is a dark headphone with a treble rolloff which is a little too aggressive for my tastes. It is hard to tell if the P7 rolls off, but it is less than the P5 for sure. It has a balanced sound, with a bit of a boost in the bass. In fact, the bass sounded loose to me. I was listening to Diana Krall's "Live In Paris" CD (one of the selections they had of which I am familiar), and the bass was a bit more forward than I am used to. Now, this might have been the fault of the amp setup, but I could not change that (I asked, and the manager said no, we can't disconnect the system for individual test).
Interestingly, they had a PSB M4U2 in a box sitting right at the station, but it was not hooked up. I would have loved to hear that headphone, but I did not ask having been shut down on the source question (and this was at a special "private shopping" event, too). In my opinion, Best Buy Magnolia's headphone listening station is basically non-consumer. About the only thing you can do is try on headphones for fit. Sound listening is a crap shoot, and there are too many constraints in the way one listens to make it remotely useful. If they were to become a bit more Japanese in their thinking, they would go a long way -- think Yodobashi Camera, folks.