Had a 9670 before.  Been trading combines every 2 years for the past 3 machines.  Pretty satisfied with the S660 and the new flex draper.  Had a few issues this summer running rye, but overall, few complaints.  Not overly satisfied with the yield monitoring system on it, though.  Had been running Ag Leader Insight on other machines and switched to the 2630 on this one.  It had trouble holding the calibrations from day to day and made a mess of our yield maps.  One day it would be right one, next day 10% low, next 10% lower, next 30% high, and next back to right on.  Frustrating and makes my yields maps almost completely worthless for the season.  Dealer wasn't a whole ot of help with that part either.  I will be switching to a different system for next season, maybe Precision's new setup.

As far as comparing the 660 to the 9670, I would say the 660 has more capacity.  My biggest complaint on the 660 is the cab noise in corn.  You may have seen me post about it during harvest, but its awful.  Can hardly talk on the phone while shelling and by the end of the day I'm glad to get out of the machine and find a quiet place.