I haven't heard the D600, I have demo'd both the K550 and P5 side-by-side. I felt the P5 was better built and better looking, easier to get a good fit/seal, and about equal in sound quality albeit more expensive. It's more user friendly. To throw a wrench into the works, I also tried the AT ATH-ESW9 (which I ended up going home with) - better fit, better sound, better looks (to me).
The P5 seemed to isolate better than the K550, but I can imagine the K550 being more comfortable if you get a good fit/seal. Sound-wise they're both good, as is the ESW9 - really depends on what you like though. The P5 are kind of dark (but not massively bassy) and can be a little bit "whizzy" or "tizzy" in the upper midrange (resonant would be a good word there methinks, the K550 felt somewhat upper-mid/lower-treble tilted with less bass but larger sound-stage, and the ESW9 had more coloration through the mids (which I happen to like, but ymmv), at least equivalent bass impact, and are relatively rolled off but crisp treble.
I'd add a fourth headphone to this comparison, the Kenwood KH-K1000, which are a fantastic full-sized closed can. Better than the Denon AH-D2000 imho, but no idea how they would (or wouldn't) stack up to the AH-D600 (which are a can I've been curious about, admittedly). They have a more even FR than any of the above, great extension, less coloration, large soundstage (they have angled drivers), and are very comfortable (albeit heavy). Not a lot more you can ask for in a full-sized closed can methinks (except for lighter weight perhaps).
I think, again Denon D600 out, someone could be happy with any of these four. It really comes down to preference and which presentation (or comfort/fit) speaks to you best. The AT, B&W, and AKG cans were all available to demo at Best Buy - the Kenwood aren't available in the US except online, so they're kind of a "sight unseen" purchase (which I do realize represents risk).
I think the K1000 are the most comfortable in terms of pad fit, but the ESW9 in terms of long-term wearing (they weigh about half of what the Kenwoods do!); the P5 and K550 are probably somewhere in the middle. The P5 seemed to isolate best of pack, and are (if memory serves) the only ones with removable cables and ControlTalk (I know the ESW9 and K1000 lack that feature, I can't remember if the K550 cable comes off or not though).
Also, if you go with the ATs or B&Ws - I'd look to go through an authorized dealer; I know the ESW9 are one of the most counterfeited headphones in the world, and I assume the P5s are probably up there too (they're popular luxury audio after all).