I come from a somewhat different angle than a lot of the people on here...I'm more of an audiophile used to listening to full-range speakers, than a head-fi'er. When I listen to headphones, it's exclusively on my portable, unamped iPod/iPad/iPhone. So, I'm generally looking for closed-back, easy-to-drive headphones that do the best job of emulating the experience of listening to my HiFi as possible, given those limitations.
About B&W...first off, while I respect B&W speakers and acknowledge that they are a true speaker manufacturer and not an imposter, I have never found any of their speakers to be quite to my taste in any given price range. I'll frequently listen to B&W's and enjoy them, but not in the context of the price that is charged - there's always something I like better. This applies to the 800 Diamond series as much as the lower end stuff. That being said, I was very open to the idea of the P5's since the wide-open, relaxed tonality that I associate with B&W would have been ideal for headphones, and the price isn't outrageous. Unfortunately, I was sorely disappointed - these headphones sound nothing like B&W to me. They're congested in the midrange and bandwidth-limited at the frequency extremes. There's no subtlety, just a kind of slodginess to the sound. This is all coming out of my iPhone, with uncompressed ALAC files playing...compared to the Bose and Beats headphones that also grace the Apple Store floor, the P5's were clearly worse than either of those headphones, and it wasn't close to me (and Bose & Beats aren't exactly HD-800's). I suspect that the P5's, despite their being designed for portable players, really need an amp to sound better, as they exhibit symptoms I'd usually associate with under-driven speakers.