Yep I'll agree with you, I am often on a plank by myself when it comes to Spike. I think that nothing has changed him, don't think he has grown at all, but I ahve written this and defended it so many times I won't bother again.
As for the nature of a vampire in Buffyverse. Forgetting the arguement that if vampires can change it makes Buffy a mass murderer (and there is nothing to suppose that Spike is anything special as a vamp goes), I will quote from Psyche's shooting script from Loyalty, Angel is talking to Aubrey:
Angel: "When somebody becomes a vampire there is no turning back. (Wes looks over at Angel) No matter how much you want to believe there is some part of him you can save, all that's left is an evil thing."
I think thats a pretty obvious statement as to the nature of vampires in the Buffyverse. I actually think it was put there for all of the Spike redemptionistas. Vampires can't change (well without supernatural forces). BTW I think when Spike comes back from Africa, he won't be an evil vampire, but thats only speculation.
