I loved season 6. It's definitely not my favorite, but it's up there.
My favorite part of the season was probably the Spike/Buffy storyline. It's weird, because at first, I really didn't like their relationship in season 6, but after rewatching it, I love it...in fact, unlike most people, I preferred season 6 Spuffy to season 7 Spuffy. Now, I know a lot of people argue that Buffy's actions were out of character, I disagree...she had been slipping down the path of self-loathing for some time (just look at her having to go on the spirit walk in season 5 because she thought she was becoming too cold and closed off), and going to heaven before being yanked back to Earth just heightened it. It made sense that she would take out her anger on Spike, since he appeared to be nothing more than "a thing" to her. Their dynamic was just so complex, if not a little unsettling at times. Plus, hello? Lots and lots of hot sex!
I can see why people a lot of people don't like it, though. I think the main reason is its difference from the other seasons. It can be very heavy and mature at times, but I liked that aspect of it. It fit the Scoobies' positions at that time, even if at times, the "life sucks" aspect was taken so far it was borderline ridiculous. Then, there's the whole thing with the metaphors losing their subtlety, which I can sort of agree with, mostly in regards to Willow's arc. The "magic addiction" plot did fit the character's position at the time, since she had a weird almost obsession with power since the beginning of the show that gradually became more prominent, but I thought it was poorly executed. The whole "MAGIC=DRUGS" message was smashed into our heads so much that the storyline was completely ruined.
Still, the complaint that I just don't get about season 6 is that the Trio were lousy villains. They weren't SUPPOSED to be the Big Bads, or even a real threat...notice that, whenever Buffy was pitted against them, she effortlessly sent them heading to the hills. The whole point was that life was the Big Bad, and the Trio trying to mess with her life were just icing on the cake. Besides, I liked the Trio. They were hilarious and a real breath of fresh air amidst a very dark time.