You know, I've thought about this very issue a lot myself. Discussed it with some friends. Thought about writing up an essay of my own.
Much as I've loved and liked James Marsters as Spike over the years, the longer the character has remained on Buffy the Vampire Slayer, the more wrong it's seemed. I've often wondered what the show would be like if Spike had been dusted or left town a season or two or three ago. I really think that were the character (and actor) not so wildly popular, if it were just about story-- that Spike either would've been killed or else would've sacrificed himself by now.
Jaime J. Weinman's article "Why Spike ruined Buffy the Vampire Slayer" is a fascinating read. (Note that it mentions events through last week's episode). It's at salon.com, so you'll have to sit through some ads if you haven't yet signed up for Salon Premium.
If you're a Buffy fan, you'll probably want to be visiting Whedonesque regularly to keep up with all the coverage of the end of the series.