Years ago I used FS2002 to shave many hours off my IMC Rating training. It isn't at all accurate in pitch behaviour and one sometimes ends up trimming the plane using power instead. The real plane is usually much easier to fly. But it's very good for procedural stuff; you can set up a flight from Heathrow to Hong Kong and it can be very real.
My 11 year old son now plays with FS2004 and FSX. FSX is very good but the controls (without force feedback) are still not even close to real. I did try a FF stick a while ago and it was cr*p.
However, after many hours of flying all different types, the other day he ended up in the front seat of my TB20 and flew it perfectly, to a very good PPL standard, headings and altitudes, steep turns and holding altitude to 100ft, and then flew it all the way to the runway. I just had to pull back on the yoke a little to make it flare, but the landing would have been fine safety-wise anyway. Quite amazing. I did the landing gear and flaps for him.
FSX is pretty amazing. If they got a force feedback stick/yoke to work properly, it would be very realistic.
A sim is however difficult for VFR approaches because one doesn't have the side/down view and the same time as the front view. This is crucial; could be achieved with multiple monitors.