You can't go wrong with a 660 thats in nice shape. The 660, 664 and 688's were all essentially the same baler with incremental improvements with each model. About the only real thing to ask about is to see if the 660 had the slip clutch update put on it. If not, plan on $500 or so for the update. Otherwise, you can bale 8 - 12 mph while cracking out 2000 lb bales. Good balers.
Rod