I don't have on an AR, but I do have the Fastfire II on a .22lr pistol. It is the least expensive, <$200 red dot I'd put on a gun. In this price class it is very good. I've tried many of the cheaper ones and none were satisfactory.
Here's the good and bad with the FF II. It has no external on/off or brightness controls. Good, because you simply take the cover off and the dot is there. It adjust brightness based on ambient, automatically. Put the cover on and it turns off for storage. Bad, because at some point, in low light the dot turns off. In very low light, but where the target is illuminated, you won't have a dot to work with. I'd only use FF II for reasonably lit target, competition situations.
Now they have the F3 and the AR F3, which seem to have a manual controls, which over comes the the low light problem. I have not tried theses, but assume it's the same basic red dot just with manual controls. But then you have to remember to turn it on to use and off when done. There's just no free lunch, is there?
Hope that helps.