I think a sim is a great way to supplement one's learning. I used FS2000 (as it then was, with fairly crap scenery) with add-on scenery for UK airports and surroundings, and I used to do x/c flights on it the day before doing them for real, comparing the scenery against the map as I went along. It was a real help.
It's no good for learning landings.
It's brilliant for instrument flight. One should never go airborne for any instrument stuff until one can do it all on a sim.
Today's FSX is a lot better than FS2000 was. My son has hundreds of hours on FSX and flies the real plane very well too.