I hear Adams Arms is all right but no first hand experience. But I wouldn't buy one since $600 buys a really high quality direct impingment AR upper with absolutely no doubt that it is good to go.
Whichever way you go, do a lot of research first and on the benefits of piston vs. DI.
Don't forget barrel profile, weight, length, twist rate, etc.
Last edited by Quentin; 10-18-2010 at 05:38 PM.