Spike's long-term introduction in to Angel's world was the same for when Angel got his own show: once he was away from the one he loved he became a newer person, one who ended up doing things on his own and growing much more than before.
I adore Spike. He's probably one of my favorite things about both shows. And I love when he's brought in as a member of Angel. As someone else said, we get the chance to see what having a soul is really like for a vampire, in the very early stages when it's still all new, the ups and downs, the choices. He really didn't have much chance to grow beyond the beginnings of the torture of it all in Buffy, season 7, because he spent the time being crazy, and being used by others around him--not to mention finding his footing with his new soul, and just where he stood in Buffy's life. In Angel, he was finally able to branch out a bit.
I loved the dynamic between him and Angel. They teased and annoyed each other in a way that just seemed so natural to me. He was in a place where he didn't exactly belong, and wasn't exactly wanted at first, but his presence there did do season 5 a great justice, I think just for that contrast between Spike and Angel.