I liked Wood's character. He had issues, but that's not surprising. Having a slayer as a friend or sister is hard enough, but as a mother...? Your mother should be the one person in the world who you know would put you first, beyond everything, but Wood knew that wasn't quite true: the mission was what mattered. Plus, if Spike was right in Fool for Love, and part of Nikki wanted it all to end, then her son wasn't enough of a reason for her to stay.
I can completely understand Wood wanted to kill Spike. Finding him and getting revenge had been the focus of his life (maybe at the expense of indulging in some healthy grieving and moving on). A few minutes of thought though, should have made him realise that, big picture, they were in the middle of a war and Spike was one of their strongest soldiers.
I think Wood also had a slightly oedipal thing going on with slayers. He wanted to be close to Buffy and Faith, because it was a way to be close to his mum.
He got over the thing with Spike (well, enough to be able to fight on the same team) and threw himself into the battle, so he deserves some credit for that.
Plus, he was really funny! Delivered some great lines!
Bringing Wood in was a really interesting way to resurrect Nikki's character, which always stood out to me, even though she had just a few minutes screen time back in Season 5. I think it would be interesting to find out a bit more about her (A comic perhaps?) . I assume she didn't get pregnant on purpose, but decided to keep the baby, and then managed to combine motherhood and slaying for four years....