There are those who promote the gas piston guns as a better mousetrap and there are those who claim it answers a question that was never asked. My perspective is that the gas piston might be a better option for the individual willing to pay for it, but not if the additional 30% cost prevents you from purchasing a rifle of equal value as the DGI (direct gas impingement) gun you would otherwise own. While I'm sort of in the same boat still trying to define the AR I want, I believe I'm closing in on a choice.
The AR was designed to do away with the gas piston of older designs like the FAL, G-3 and CETME rifles. But the result was a rifle action that heats up and badly fouls the firing components within the receiver once the lubricant has burnt away. I know this from firing then cleaning C-7s while on a Ranger course, and the cleaning that followed the firing involved scrapping carbon off the effected components, which over time I'm sure limited their life expectancy. The solution I've been hearing is to keep the bolt wet with lubricant, the idea being that the carbon can't stick to steel that is wet with a good quality lubricant. What we found on the ranger course was that through extensive full auto fire, any lubricant that was present burned off and the result was carbon fouling that proved difficult to remove. What the gas piston does is keep the heat and fouling at the gas block ahead of the forearm allowing the gun to run cooler. The disadvantages are that the heat is more focused in a smaller area, the heat might damage any plastic accessories you have installed near the front of the rifle, the gas block of the piston rifle is bulkier and heavier than the one on a direct gas impingement rifle, and the area around the gas block in front of the hand-guard of the gas piston rifle can become very dirty from fouling.
The chances are you will never heat up your own expensive rifle the way we heated up those C-7s, and regular maintenance will keep the functioning and condition of your rifle in top form. While I disagree with those who say that a gas piston is a complete wast of money, I will say it probably does not represent good value for most shooters considering the additional cost. Now myself, I'm still on the fence, but my choices of gas piston gun has limited itself to ADCOR's B.E.A.R which has a gas piston directly attached to the bolt rather than a piston which impacts a striker plate on it, while there are a number of DGI guns I'm still considering.
Some issues to consider before deciding on which gas system to choose are: the temperature extremes you will likely use the rifle in, the number of rounds the rifle will digest between cleanings, and the types of ammunition you intend to shoot.
