My brief thoughts on these 'phones (dealer's demos, no idea on burn-in):
I finally got a chance to listen to these properly. On first glance, the build quality is fantastic. Very supple pads. I do find the fit to be comfortable, but not secure. It's likely that I would have to adjust the headphones many times during the course of a day's mobile use to make sure the earpieces still sit over my ears. The sound is much better when the fit is good.
The highs are detailed and extended, if not slightly etched. I don't feel the it's well intergrated with the rest of the spectrum and certain jumped out at me on some occaisions. The midrange is generally on the warm side the lacking transparency found on high-end headphones. The bass is tight... ish; bass extension is OK, but all-in-al I would've wanted a bit more bass prominence. A good fit is necessary for good bass. Soundstage is reasonably spacious but there's not much in terms of height. I forgot to listen critically for soundstage accuracy and hall/room ambience cues. In general I would've wanted a bit more refinement and micro-details. I had to turn up the headphones to really hear all the fine nuances in the music. Great headphones sound spectacular at low volumes and I'm afraid the P5 isn't one of them. That said I might be asking a bit much since B&W never marketed these as reference class headphones.
This is a closed-back headphone relying on passive noise-isolation. I must say the noise isolation was decent, but perhaps not as great as more humble headphones like the DT770 and dare I say the Bose Triports (or whatever they're called nowadays). Sound leakage might be an issue too, but it isn't too offensive.
For the price, yeah you'll be able to find better sounding headphones/earphones for the money, but the whole product oozes class and actually sounds decent. I would recommend an audition to make sure the fit is good before purchasing.
The bottom line: Decent effort from B&W. Natural sounding, classy-looking headphones that are also enjoyable to listen to. Might get a pair myself.