I just received my 650's, which are my first pair of Sennheiser's, something I have been looking forward to for awhile after seeing all the praise here on Head-Fi. My immediate impression is that they sound great. I'm just using a cheap little SMSL that I just got, so not sure if I'm getting the full effect yet. Later tonight I'll try them at home on my Bifrost + Vali combo, and see how they sound there.
One thing I'm noticing right off the bat tho, if I have them actually sized to fit my head snugly, they REALLY hurt at the top of my head. I have to loosen it up a bit, which isn't really a problem, just surprised me how tight they can be when actually fit to size.
I have pretty big head (please no jokes😄) and my 650's are extremely comfortable. It took about a month of use (2 hours a day) for them to loosen up, enjoy!