I did one ages ago as a requirement for a class I was in (actually, I won it). I wouldn't really bother now as I think contests tend to be more difficult to win and not mean as much as getting a short story published in a magazine, for instance. I've considered a few novel contests just because if it's relatively small I might have a better shot, but I still don't think I'd win. I don't think my writing is good enough to even necessarily make the top five. I don't think I've got a bad book by any means, either. I just don't think it'd be good enough to be best, and if it's not best it's basically meaningless as a credential.
If you want to enter just for kicks to see how you stack up go for it, though. It could be fun.