I have both the D5000 and K550 among many others and I feel the K550 is overall the better headphone IF you can get a good fit. If they don't fit right they can sound really bad which is why I think you see such mixed reactions to them. I really only like the D5000 for electronic music where its extra bass is fun. It is true that it does not seal and besides leaking sound it also provides basically no isolation. A D5000 on a plane would suck, they really are not a portable headphone at all. The K550 isolates much better and is more suitable as a travel headphone although it's pretty large and the cable is too long. I use both with a Fostex Hp-p1 as DAC/amp for the times when I want to get away from my main setup. When I travel I take the Fostex and the K550.
I haven't heard the new Denon D600 but I do have a pair of the new D7100 and I feel those are very good headphones. Vastly better than the D5000 when used as part of a high-end system. The new ones are also sealed and isolate pretty well so if you're looking at a Denon alternative the D600 would probably be a better candidate than the D5000.