I've had the P2 for a few days. I also have a P1, which I have used religiously for the past 2.5 years. I'd like to outline some of the differences as I see them between the P1 and the P2. I don't have the other Cowon DAPs (M, M2, PS), so can't comment on the differences or lack of, between those models and the P2.
The P2 is a noticeably chunky device, larger and heavier than the P1. This size difference is easily noted in the hand.
In terms of the hardware, the P2 is a definite upgrade to the P1 in almost every respect - the screen is nicer, with a glossier finish and blacker blacks. It has a volume knob (and another knob of dubious merit), and a multifunction button (again, somewhat spurious as it duplicates already easily accessible functions). There is no flappy flap over the micro USB socket (this is my favourite change). The P2 has balanced out, if that is important to you. Its case is nicer (with the exception that the covered hardware buttons on the side are more difficult to control by feel alone than the exposed buttons on the P1). Overall, I find that the fit and finish is reminiscent of the 2nd gen AK devices.
The P2 has also had a minor UI refresh (this may be the same as the PM2 and PS - I don't know). The icons have been refreshed, there is a new "Now Playing" skin with less skeuomorphism, and the boot-up design has changed. The P2 seems to flip into landscape album cover view at the slightest provocation. I have to wield my P1 aggressively to produce this view, so looks like the gyroscope has been changed or reworked.
In terms of sound, there is a definite difference between the P1 and P2, which I was relieved to hear (I didn't want to realise that I had spent €1100 on a volume knob). The P2 is voiced differently, presumably a result of the change in DAC. While it is still very much a Cowon device - very musical, tending towards warm, detailed but not analytical, the P2 is a step up from the P1. It sounds slightly more forward, with more pronounced low mids, and more focused and prominent low end. This does lend it a "meatier" feel than the P1, but still remains a very smooth and enjoyable listen (I use W60s and Sennhesier Momentums Over Ear v1.0, both unbalanced). The soundstage is modestly larger.
Overall, I'm happy with my purchase. The device reeks of quality and craftsmanship, has a very stable OS with fast boot times, and sounds like a high-end Cowon device (and a modest upgrade to the P1). It would have been nice to have USB 3 / C and bluetooth, but then, I suppose, it wouldn't have been a Cowon device...