People tend to not like them vs Grados, which has a very forward sound.
I prefer the HD650 (I actually EQ my HD600 to mimic its response above 1000hz, flattening the 3500hz peak) though since very forward headphones tend to sound like they're throwing everything at you on fast music that has a lot of instruments, like Dream Theater, Epica, Nightwish, etc. HD650 lays the instruments out a little farther from the inside of your head, though not necessarily far enough outside of it as with the K70x.
Basically, as far as headstage and perception goes, it's not anywhere near standing front row. It's more like if the aforementioned bands played in an orchestra hall and you're sitting in the center of the main section in front of the stage.
As for dynamics, particularly with the percussion, one other way they're different is how much easier to drive Grados are on top of the low impedance. HD6xx have a high enough sensitivity/effciency, however, if you plug them into anything that lacks voltage output at their impedance, or starts to pile on distortion, you get a sound that's basically a snoozefest. Plug them into a decent amp and as long as the output is more or less equal they're not too far behind Grados. I had the HD600 and SR225, and due to some high output impedance gear I was plugging them in at the time that made the low impedance Grados sound like tin cans (my Marantz CDP, NAD 304, and LD MkII), I kept the HD600 (note that while the Grados did fine out of an active ground CMOY, it sounded dark and lazy with the HD600).
If you get an amp and listen to prog/symphonic, I won't recommend against trying them. If you will either not get an amp or you don't listen primarily to symphonic/prog metal, skip them. You might as well get a Grado SR125e or SR225e.