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in theory its suppopsed to keep the gun cleaner and run cooler. IMO it's the flashy new hottness and will not replace the gas system any time soon, it's just one of system of moving parts to go wrong. At the cost of a piston driven AR you could build 2 comparable gas AR's or buy one and have $ left over to tacticool toys, ammo, or range time.
The reason there are so many Ruger upgrades is because they're necessary.
Huge waste of $$$, the direct impingement design is well proven and it really doesn't even need to be all that clean as long as the rifle is lubed. People who complain of failures in an AR either have a screwed up rifle or they aren't lubing it properly, they tend to like to run on the wet side.