Would I watch a Spike spinoff? Maybe. It would depend on what exactly Spike would be doing, where he would be doing it, and whom he would be doing it with.
I don't think I'd watch it if it was just Angel but with Marsters in the lead role. If they came up with something new and interesting for him to do then sure, otherwise the show just becomes Angel2.0... been there, done that, got the T-shirt (okay, I don't actually have the T-shirt, but many other fans do). It would have to be something new and befitting Spike's personality. Something involving revenge perhaps? Maybe Dru could be involved?
A change in location might also help sway me to the show. I'm getting a little tired of seeing demons traipsing about sunny California. Maybe a visit to the Cleveland Hellmouth? A trip for Spike back to the seedier sections of merry old England? I'd kind of like to see someplace else. After eight years of the Whedonverse hanging it's hat in Cali, it's just starting to make the State look like a good place for the suicidal to move to (I mean seriously, why does every apocalypse start in California?).
It would also certainly depend on who's in the supporting cast. I'm not sure if Spike is a strong enough character to carry a show on his own. The character generally lacks direction. I could see a Faith show being a one character thing, or perhaps even a Lorne spinoff going in that direction. I don't think the other characters, from either BtVS or AtS have the ability to do so. Most either don't seem to appeal enough to fans to carry their own shows, or the characters simply don't have enough of an independant nature to be on their own. I think a Spike spinoff, like BtVS and AtS, would have to have an ensemble cast.
Any Spike spinoff ideas you'd like to throw out?