Spike on Angel is, unfortunately, Diet Spike. He is shot to look shorter, his accent is different, and his voice is pitched higher. Gone is the Spike who draped himself decoratively over a cross.
However, Spike is a breath of fresh air on a show that has boxed itself into a very depressing corner in Season 5.
A little background: Angel sired Drusilla, who sired Spike. Angel is no longer the only vampire with a soul, as Spike recently got himself one too. There is a prophecy, the Shanshu prophecy, that states that the vampire with a soul, if he saves the world from a bunch of apocalypses, will become human. Now there are two. And both of them have dated Buffy. Tricky.
Of course, for the first half of Season 5, the point is moot since Spike is a ghost (he died saving the world from an apocalypse, but somehow reappeared in Angel's office):
Fred: "Spike has a soul?"
Angel: "Forgot to mention that."
Spike: "Or maybe Captain Forehead was feeling a little less special. Another vampire with a soul in the world."
Angel: "You're not in the world, Casper."
Things get a little more complicated--and violent--when Spike gets magically recorporealized, and realizes that he just might be the vampire with a soul the prophecy is talking about--a theory that sends Angel into a tailspin. The two of them go after The Cup of Torment, which will supposedly determine which of them it is. (For anyone who just rolled their eyes at the lameness of "The Cup of Torment," it's a joke: the thing turns out to be a hoax, and filled with Mountain Dew.) Spike an Angel beat each other up pretty badly for two guys who are supposed to have souls and be fighting for good and all, but the situation seems to be bringing up some unresolved issues, as it were. Back in the day, Angel was just about the worst kind of serial killer you can think of, so bad that he could atone for a thousand years and save millions of lives, and probably still go to Hell. He emotionally tortured Spike, and turned a kind of happy-go-lucky nerd of a vampire into a mass murderer. He was cursed with a soul, as punishment; Spike nearly died earning his.
Spike: "Every time you look at me, you see all the dirty little things I've done, all the lives I've taken--because of you! Drusilla sired me, but you made me a monster."
There was one episode I almost didn't rewatch; it's great, but it's very scary and I was all alone.
In the last episode of Buffy, a bunch of girls known as "slayer potentials," i.e. girls that could be called to be a slayer if Buffy were to die, were suddenly imbued with slayer power, creating an army of slayers. One such girl, Dana, was in a mental institution--she had been kidnapped and tortured as a child, and was already not what you'd call stable. When she received the memories off all the slayers that had come before (a side-effect), the violence of these memories triggered a true psychotic break. She escapes and begins slaughtering everyone she comes across. Because Dana's memories include being murdered by Spike (he killed two slayers), Spike gets special treatment: she cuts his arms off.
Spike (to Angel): "Lass thought I killed her family, and I'm supposed to what, complain because hers wasn't one of the hundreds I did kill? I'm not saying you're right, because I'm physically incapable, but for a demon I never really did think that much about the nature of evil. I just threw myself in. I liked the rush. I never did look back at the victims."
Angel: "I couldn't take my eyes off them. I was only in it for the evil. It was everything to me. It was art. The destruction of a human being...I would've considered Dana a masterpiece."
Spike: "The tingling in my forearms tells me she's too far gone to help. She's one of us now. A monster."
Angel: "She's an innocent victim."
Spike: "So were we. Once upon a time."