I love S6. It's in my top three favorite seasons. And I'm probably going to end up repeating some things here, but with such a long thread, won't we all? XP
The Spike/Buffy relationship makes sense to me. Did I like it? No, I felt horrible for Spike and annoyed with Buffy. Was it disgusting? Yes, but it was supposed to be. But did it make sense, how it happened and why? Yes. Buffy was emotionally numb this season. Being ripped out of heaven left her feeling nothing...and people do some pretty weird things when they feel nothing. As she said time and time again, she just wanted to feel SOMETHING...having sex with a guy that's in love with you, even though it completely disgusts yourself, is definitely "something" to feel. Was it right of her? I don't think any of us can say using somebody is "right"....but Buffy didn't think it was right, either! She knew it was wrong, she hated herself for it, but even that was better than feeling nothing. I think the writers definitely wanted the relationship to bother/disgust us. That was the point.
I do think the Scoobies were too hard on Spike, but at the same time I don't think you can say "But Angel was evil once too!" Angel with a soul is not the same person as Angelus without one. With a soul, Angel is a new man, a changed man, he has a conscience and a moral compass. Without one, Angel is dead, and the demon Angelus wears his face. Spike with a chip is still Spike with a chip. As Buffy once said, he's like a serial killer in prison. He's a vicious dog held back on a leash. Or, at least, he was this at first. It's been pointed out already, but what was the first thing Spike tried to do after thinking the chip wasn't working? Vamp some girl. Angel would never have done that...Angelus maybe, but not Angel. And I don't really see where you're getting that Angel was more forgiven than Spike. I don't think Giles ever truly forgave him. I think he tolerated him for Buffy's sake, but we have no evidence to suggest that all was right between Giles and Angel. And Xander? Xander is and always has been a total jerk where Angel's concerned. Willow...she basically behaved the same toward both Angel and Spike. She wouldn't even let Spike stake himself, remember? Now, granted, I do think Spike became "less" evil...but I think the Scoobies' problem with this was, it was all because he was being held back. He's violent by nature and he could only attack demons, so what choice did he really have but to fight beside Buffy? They may have been unfair at times, but if the chip ever did stop working, I don't think any of us could say for certain Spike wouldn't start killing humans, especially during moments of rage or frustration. I love Spike as a character, but even I could admit that. You can't compare a government chip to a soul...it actually annoyed me when Dawn made the comparison herself.
I also think Buffy was extra hard on Spike because, as I said, she was disgusted with herself. She was putting all her feelings about their relationship and her actions onto him. "You're disgusting, you're bad, you're nothing." And I agree with whoever said that when she was pummeling him, she was actually pummeling the part of herself that was behaving that way.
Season Six was totally about growth and development and growing up. It was all about fighting inner demons (Xander's feeling about marriage and fear of ending up like his parents, Anya becoming a vengeance demon again, Buffy/Spike, Dawn and her kleptomania, Giles and his torn feelings about being a father figure, Willow and her addiction to magic, etc etc.) That's why there wasn't a "big bad"....each of them had their own unique internal big bad to fight. I think the season is brilliant, especially since a lot of episodes and themes make us so uncomfortable. That's what a truly great TV show, movie, book, whatever should do. Make you wince, and come back again to see what happens next.