Stay away from this model, or at least, be very cautious if you do.
http://www.torontoairsoft.com/ACM-AC...ht_p_1072.html
I have this one, and while it looks good, it does NOT have the fiber optic, therefor, no illumination whatsoever. The front post sight is not straight and the reticule/crosshair whatever you want to call it was angled when I got mine. Granted that one is an easy fix, but I did find it hard to zero it. Also, if your going to use it for a GBBR, make sure it's on really tight, otherwise it will loosen and maybe fall off (almost happened to me).
The doctor sight works great though, I'm using it on my SCAR right now. I've zeroed it once and even though I haven't fired, I've taken it on and off and it's pretty much maintained it's zero. Only times I've really had to adjust it is when mounting it on a 45 degree angle and now because I mount it fairly far forward on my receiver.
Maybe I was just unlucky, but I'm going to stay away from these A.C.M. clone sights from now on...