I'm actually in a process of buying new headphones myself, and HD600/650 or Q702's are headphones I'm considering. I spent about 2-3 hours total listening to each one in different stores and at friends who have them. They're great headphones, but I was a bit disappointed when I heard each of those for the first time, because as I said in my first post, they didnt make me go WOW like HD558's and 598's did when i listened to them at times when senn hd212 were the best headphones i've ever owned.
However, I probably WILL buy them, or some other pair in 400-500 euros range. Currently waiting for first Beyer T70 and Sennheiser RS220 reviews (which are apparently excellent and should be equal in terms of sound quality as HD650's, without the need for amp of course).
I'm also worried about the fact that HD600/650 have been around for a very long time, and Sennheiser has been updating pretty much all of its headphone lines this year, so i'm expecting 600's and 650's to be updated or replaced by new models pretty soon as well.