Any sim is better than no sim.

Even flying an old version of MicroSoft's Flight simulator using a 1980's style cheapie joystick can be helpful in gaining some "feel" for flying the models. Simply set the sim to "Tower View" and select a Cessna 172/182 or similar airplane and its very much like flying an RC trainer.

FMS is not bad, and is usually the sim that they sell on E-Bay wiht a TX adapter for about $15 to $20. The TX adapter makes it a passable deal. FMS is free for download (shareware or freeware... I forget which) and includes instructions for making the adapter.

RipMax makes an OK sim wiht a set of tasks to try to complete. The tasks are not easy for the average sport modeler to do to the satisfaction of the program. (If you can do them all the way the computer wants you to... you are doing very well.) Graphics are not as good as Real Flight G2 and aircraft selection is limited. This sim shows you the debris field when you crash. (its very hard to have debris cover the whole runway.)

Dave Brown's flight sim is good. It gves a repair cost estimate when youcrash. Its possible for it to tell you it will cost $600 to repair the $300 value trainer. (if you go straght down full power and try to drill a hole to China.)

RealFlight G2 is about as good as the sims get. But the realizm ends when the model hits the dirt... it just bounces some and then starts smoking.

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Obviously... its somewhat entertaining to see how big a wreck you can make with the sims that show the results. But remember that preventing the wrecks of the models is the whole point of using the sim as a training aid.