I actually really liked Season 6 - I know alot of people disliked the whole Buffy & Spike relationship too but I actually understood it, and what they were trying to get across. Yes it was out of character for Buffy, but then don't we all get a little crazy from time to time? There were some really good eps in this season, but then for me it would have to be really bad for me not to like it.
It was just a much darker season, that was all about the scoobs growing up, and taking responsibility for their actions, whether they be good or bad. I think they reflected that pretty well.