I have been playing for the last several months with a red-dot on an AR. Not just any Red-Dot, but a Holosite.
I know, I know.. "Rob.. how could you go out and buy a Holosite for your AR??" well, I was given a couple to use in a rimfire event last year, so I took one off of an M-41 and thought I'd play around a little.
I am madly in love. In fact, I've been working with a couple of experienced CQB/entry guys to develop a system for training specifically to the set-up. (in fact, see the recent "RBD" thread in rifles for the exact set-up).
We have been mouting the weapon low on the shoulder. We sight them in at 25 yards. The reticle (dot in circle) sorta floats in your field of vision, at the top of the Holosite's box. Keep both eyes open, move, scan, hop, jump, find your target-- bang. Keeping both eyes open the whole time. It is the closest thing to point shooting a rifle that I've ever found.
Fast, multiple target hits, Cooper-esqe "snapshots" and shooting on the move are what we are focusing on.
Head shots at 25-30 yards? 5 tactical teds spread in a box 15 yards long by 5 yards deep... consistently less than 4 seconds from a low ready.
Need a long range shot? Drop your cheek down to the stock and use the iron sights.
There is a PSD team that is seriously considering adopting the Holosite for their M4s, so we have been working on a lot of movement, deployment to and from vehicles and multiple target drills.
I was especially surprised that the Holosite hasn't fallen apart or gotten knocked off. It is a lot more rugged than it looks, IME.
The dot's brightness is adjustable for day/night/indoor use. The only complaint I have is that it automatically is at its brightest setting when you first turn it on, which can be pretty glaring at night or indoors.
The on/off button is easier to operate than the knobs on most red dots, IMO. You can push it easily with either hand while the weapon is shouldered.
I played around with an ESD traditional Red-Dot and I didn't really like it that much. It was much more obstructive of the field of view and you had to have your eye directly behind the tube to get a good picture.
The Holosite's list price is between $300-$350, but I think they can be found for less.