I also enjoyed quite a lot of Season 6...there were some bad episodes, but some of them I liked, and I found the Spuffy storyline fascinating; although I didn't always enjoy their abusive relationship, it still entertained me throughout the season. Spike's beginnings on the road to redemption with his soul were also great. Also, although there was the abuse, and the violence in the relationship, I think it did have a lot to do with their true friendship and trust [perhaps even love] in S7; even though we saw the violent sex side a lot, there were times when we saw Spike support Buffy, and act like he truly did love her. There's the way that Buffy can only tell Spike that she was in Hell; his distress at not being told that the Scoobies brought her back, and us learning that he fought with them all summer; and of course, his "every night I save you" speech.
I think it was mainly the character developments we saw that I enjoyed the most; everyone had to grow up and become a lot more adult, and deal with real, adult problems, like financial problems, relationship problems etc. It could get dark and depressing sometimes, but I think that sometimes that was necessary - life isn't all roses, and S6 showed that, although it could be a little OTT on occasion.
I did also rather enjoy "Gone", which I know wasn't all that popular - wasn't the best, but certainly not the worst. And of course, Once More With Feeling and Tabula Rasa - two personal favourite episodes of mine.