Very close to the real thing, IMO. Many "mob" movie fall short in realism (Goodfellas? Aaack...) Godfather II also shows true to life immigrant experience and how easy it was to fall into crime.
How exactly was GoodFellas not realistic? Not only is it heavily based on a true story, but many people knowledgeable about organized crime have said it's the most realistic gangster movie ever made. Although not everything pertaining to Henry Hill and the real life events portrayed happened the way it's shown in the movie, I don't see anything in GoodFellas that would be impossible or even far-fetched.
