Dark Willow was a nifty villain, and as a stand-alone character I think I'd have enjoyed her quite a lot. The moment when she absorbs all the information from the books is good, and she sure looks cool standing atop the lorry. I have a hard time saying I "like" her, though, because I really wish the season hadn't gone there in the first place. People talk about how Willow's relationship to Tara "cheapens" what she had with Oz, and there is a lot of people who feels that Tara's memory is wronged by Willow developing a relationship with Kennedy. I never saw it like that; it's natural for people to have more than one serious relationship through a lifetime. I feel that Willow loosing the grip on reality, flaying Warren and trying to end the world, is far more of a desecration of Tara's memory than Willow rebounding with a girl who is Tara's anti-thesis. Tara was such a peaceful person and showed such integrity (apart from her attempt to hide from the scoobies) that I don't think she would have wanted Willow to avenge her like that. Even if he was a despicable creep, Warren was nonetheless human. I don't believe that anything good comes out of paying for a life with a life.
I liked the addiction metaphor (even if Willow annoyed me no end with it, that's part of the package), but I wish it hadn't developed into such an extreme. I'd much rather have seen Willow spiral downwards, crash after Tara's death, and then slowly rebuild herself with the help of Xander, than to see her nearly end the world, hear the crayon speech and then have her whisked off to England and come back all insecure and responsible. (One good thing, though: I loved seeing ASW on such a lovely horse even if I'm not sure I can imagine Ripper riding.)