It all depends on the team working on it. I didn't buy Fallout 3, and still won't, because the team who wrote it and coded it didn't work on it, so I won't buy it - because, imho, it isn't a proper sequel. It'd be like reading the sequel to a Stephen King novel written by that chick who wrote Twilight. Or even worse, reading a Hitchhiker's Guide to the Galaxy Sequel written by Eoin Colfer.
So - my criteria :
If it's the same gaming style, (with or without a new engine), written and coded by the team who did the prior release - then, yes I'll buy it. Anything else - hell naw.