Pros: Varm, detailed and intimate sound
Cons: Pads could be softer / headband should be less thight
1) My setup at home: Naim DAC - Naim Headline - HD650. I love it. It's like being in a dark room sitting in the midst of the band playing - or rather inside each and every instrument simultaneously. My only complaint is that sometimes on some specific recordings I get the feeling I might prefer a slightly colder, more distant soundstage. The HD650 seems to have a slight bass "hump", which usually is very enjoyable, but sometimes can get a bit too much. Can't win 'em all :-)
I also want to add that I experienced that the 650's needed 50 hours + burn-in. I was actually quite dissapointed at first.
2) Did a comparison for an hour or so in a dealers listening-room on these three great headsets on a Lehmann Audio Black Cube Linear Amp using a rather expensive firewire-dac as a source.
1) Grado GS1000i
2) Sennheiser HD800
3) Sennheiser HD650
The 650s had a presence in the mids (vocals, sax, piano etc) the others simply couldn't match. Grado's are good - but I feel they are too bright for me. HD800 felt more bright, polite and subtle than the 650's and are terribly comfy. They all deliver plenty of detail, but the 650's make you listen more to the music...
The Lehmann Audio Black Cube Linear struck me as a very, very good amp with the HD650's. (EDIT:) They are also great wtih the Grace m902, which I presently own.