The HD650 (and any other Sennheisers I've ever heard) seem to be designed with long term listening comfort in mind. They value this over absolute sound quality and it's something I value as well which is why I've owned several and enjoyed them all. The highs are definitely rolled off to the point of making it a darker sounding headphone, but for me (I'm easily annoyed by harsh or sibilant highs) this is a good thing.
I just got my HD650s today so I won't comment on them too much yet other than to say I like them a lot, if you want accuracy above all else you can definitely do better. Headphone.com has frequency response graphs for pretty much every headphone they sell, that was really useful to me when I was trying to figure out which headphones I should try so I would go look there (you can clearly see the HD650's rolled off highs and impressive bass response for an open can).
Rambusanna, since I just got my HD650s today I can tell you that in the manual they come with, there is a paragraph explicitly stating that the HD650 has been redesigned. They're pretty vague on just what was changed, but people seem to think they're a bit less dark and slightly more detailed than before. I have never heard the old HD650, so I couldn't tell you how much they have changed.