With the 660s - if you like the 650 and Sennheiser in general, I don’t see how anybody could be disappointed with it. I can’t answer whether it’s worth the money as that usually involves private information such as a person’s income level and budget. I am fortunate enough to be able to afford it and in a time when, for example, HiFiMan can charge $6k for a headphone that seems to be built from the same materials as the $3k one and in turn the $3k is as cheaply built as the $1,300 one, the 660s is by no means a rip off. In fact, I’d say it sounds fantastic for the price.
I'm an enthusiastic fan of Sennheiser, and I'm pretty confident I'd like the HD 660S myself -- I have the HD 600 and HD 650 and like them both a great deal, and even the HD 700 which I know were far less well received than the 600 and 650, although I am less familiar with the HD 700 sound because it's on loan and I've had little head time with it so far... but the HD 660S definitely has my interest, and I'd love to hear it sometime even though it's not likely I'll buy it anytime since I'm rather saturated with headphones....
But if the 660S could *replace* my HD 650, maybe I could justify it....