I've noticed tiny tiny differences in music, Not better just different, As I've been using the 650's for nearly a week now but the X2's have much better bass to my ears and in games I much prefer the sound of the X2's, I'll keep the 650's for when I'm in the mood for classical music
The X2 has a rather large mid bass hump while the 650 is more linear(still has a hump though too) I think the HD650's will give you a better idea of what the recording is about while the X2 is all fun fun fun. The X2 does have a nice sparkle.