The original AR gas system sucks for military use. The piston system a la the M 1 and M 14 is much more reliable and the versions that place the operating spring above the barrel in the fore end are an even greater improvement.
Having said all this, I must admit that I am not rushing out to convert any of my ARs. None of these improvements have been standardized, and there could be lot of crap on the market, so I'm not sure what will really work and what won't. As mentioned before you may be better off buying a complete gun rather than performing an expensive conversion.
The real question is whether or not you really need this improvement. My ARs are used for practice and competition, and the original gas system works just fine for these purposes. The piston conversion is really only an expensive luxury for me. If I were to take an AR to war, I would prefer a piston operated system, but I'm not sure that the ideal rifle/system is available at this time. I think there are a lot of changes in store for the venerable AR/M16 and that this is a real good time to wait and see what is in store.