Just bought a pair of HD-600s a couple of months back and they are fantastic. I was thinking of getting a pair of closed-back cans for recording but saw them and thought, go on then. I'd definitely recommend them. I've heard that the HD-650s are pretty good too but I don't want to test them out and spoil myself!
Glad you liked them. Not getting the 650 is not something you should worry about. The 650's are in most purists opinion slightly bass heavy, have a tighter tolerance which only can be appreciated on very high end equipment.Of cource in the end it all boils down to personal preference.
I wish you many happy hours of listening to the Sennheisers!