Have a friend who's big into simming. Spends weeks designing various add-on aircraft for it, and regularly takes off from LHR in a 747, puts A/P on, goes to bed, then gets up and lands it whilst eating his corn flakes at LAX. He's very good at it.
Will he come up flying in the real thing? Will he chuff!
Only thing I've found PC sims useful for is instrument work. Especially setting the weather to "atrocious" and remembering how to work out drift components in your head etc..