Not a whole lot of people are using them, primarily because they have had a fair amount of bad reviews. Trouble with getting it zero'd, problems with holding zero, problems with overall durability. Not everyone has problems, and a number of people who dont like them. But too many others realize there are what they consider better, more durable optics, with better battery life for not much more money.
If you do want one of these on your AR, and its going on a flat top, get C-More's dedicated mount with the integral BUIS. It's the best one they make. If you are putting it on a A2, get their carry handle mount. If you are unsure if you still want one, get an Aimpoint! (or and EoTech...)