I have Realflight G2 Lite, which I've been using to learn to hover before going out with my Shuttle. There are subtle differences with the real thing but I can honestly say that the sim has helped me beyond measure. Yesterday was my first session and in less than two hours I was able to hold a steady tail-in hover - all thanks to the sim.

When I was reading around I felt the choice was between Realflight or Aerofly, but I'm sure others will feel differently.