I'm a huge Buffy fan, too!
I think that the reason we all like Spike so much is that his character has so much more depth than Angel's did when he was on Buffy. Spike has so many levels, he's so complex... mean, funny, tender, bloodthirsty, caring, violent, insightful, etc.
Spike also seems to understand Buffy's dark side, whereas Angel couldn't embrace that aspect of her because he was trying to banish his own (Can anyone tell that I'm really into this show?:D )
Does anyone watch Angel? They're developing a relationship on that show between Angel and Cordelia. A couple of years ago I would have HATED this, but as characters they've both grown so much since this show spun off, and I can really get behind this relationship the way I am excited about Buffy and Spike.
My only bummer is that with my new job I've been so busy I've missed about the last two episodes of each show:( )
Jen