P.C. flight sims are excellent proceedural trainers, i.e. they enable you to practice some of the basics of instrument flying very effectively which enables very rapid progress to be made in the aircraft. It is also therefore very cost effective. With larger aircraft a well written sim program can be a useful aid in learning procedures with the aircraft systems.
Using such sims for visual flying, particularly the landing phase is not to be advised as the sim does not provide sufficient visual cues, and as has been pointed out, encourages pilots to keep their eyes in the cockpit - not a good idea in a busy circuit!
P.P.