On black guys a shaved head almost always looks fine. On white guys it can look ok or not. For "not," look no further than:
http://seat42f.com/images/stories/t.../S0110/evan-handler-californication-photo.jpg
If you're in a white-collar position it's very difficult to shave it completely, because you look too extreme for the job. For example, I wouldn't recommend it for trial attorneys or surgeons.
I think it largely depends upon the rest of your face. Just like very few women look good with a short "pixie" haircut, not a lot of guys look good with totally shaved heads.