I don't know, I'm going to reserve judgment until they make it. I do think Keanu Reeves can pull off Spike, though a lot of fans are dead-set on Steven Blum's voice and the impossible afro haircut. For two decades people said the same thing about Watchmen, and it sounds like it will turn out really well.
I'm definitely more worried about who they'd cast as Faye. I've always thought of her as Asian-American (with the perfectly straight black hair) but there's no way in hell Hollywood would ever cast an Asian-American female in a leading role like that. They're always descriminated against and replaced with some 25-something WASP who doesn't fit the role.
However, there is someone I think could really pull it off. While she looks a little younger than Faye (she's supposed to be around 24, but always looked 28-30 to me),
Mary Elizabeth Winstead could probably pull it off very well.