I just ordered one and am waiting for it to arrive. My guard unit uses them on our M-4s, and I've been very pleased. They seem really rugged, hold their zero well. Haven't had any complaints, and these have seen a lot of use in OEF and OIF. They're mostly in the down position behind the aimpoints, but there have been no malfunctions that I'm aware of. I personally like them a lot. Hope that helps.