Here's my take on the gas impingement vs. gas piston debate as it pertains to AR's. The AR was designed around gas impingement - it's a good design. Compact, lightweight, efficient design. It works well. Redesigning the AR to be gas piston is making it into something it was never intended to be. I have absolutely no interest in gas piston AR's and avoid them like the plague!
That doesn't mean I dislike all gas piston 5.56 rifles - there are some excelent ones out there. If I want a gas piston rifle, I would choose one that is not AR based and was designed from the start to work with a piston.