Small update, I've had these for a decent amount of time now to form a more solid impression.
They are so far the best headphones I've owned, they really let the music come alive. Even though the soundstage isn't that big, their intimate nature and the way they handle micro and macrodynamics gives everything its proper size, so that gives music a lot of PRaT.
I'm still awestruck by how good the bass is, and I think it should only improve with a better amp.
They are the complete oposite of the Koss, where the O2 really doesn't have a distinct laid back or upfront sound signature. It really depends on what you're listening to. Whereas all other headphones seem to add a coloration to how music is presented.