I found it invaluable while getting my instrument rating.
I spent 8 hours in an "approved" simulator and got nothing better out of that, at $38 an hour plus instructor fees, than I did from FSX.
Setting up IFR flights in the sim worked pretty much like the real world and flying ILS approaches actually seemed a lot easier in the real thing. I probably spent 40-50 hours doing this while doing the actual flight training. One difference: real world instruments aren't nearly as accurate!
Many CFI's don't like sims, for obvious reasons - they don't get hours in their logbooks. Ignore those opinions - if you treat this as a training tool and not just a game for messing around, it is very useful.