I would post this on the Open Source forum but it doesn't seem to get nearly as much traffic as this one..
Anyhoo, I've been playing with the Flightgear open source flight simulator for a few months now, I used to have MS Flight Simulator but lost it when my hard drive crashed and apparently I've lost my disks during a recent move as well.
Flightgear isn't as visually pretty as the later MS offerings but the flight model seems at least as good, I've been mostly flying the Eurocopter because I've never learned to fly choppers. The plane models I've tried seem very realistic to me, I have some hours behind the stick on Cessna single engines although I never got a license and I've flown RC models a lot (which is actually harder than the real thing for various reasons). I'm using a Microsoft Sidewinder 3D Pro control and it interfaces quite well with Flightgear.
If you'd like a flight sim but don't have the money for the fairly pricey commercial products, give Flightgear a try, the price is certainly right.
http://www.flightgear.org/