I knocked one of my Kobras off my workbench onto a concrete floor...twice (you'd have thought I'd have learned the first time)...without even scratching it. I doubt you could do that with an OKO, hood or not.
There are some design issues with putting a Kobra on an AR...it was designed for an AK, after all...but it does work. I had one on my M4 for a while, and shot some very nice groups with it. If you can live with the hump in the back, which sort of forces you to look over it, then it will work fine, and they're tough as nails. As Troy says, though, a BUIS is out of the question though.
I have a ProPoint3 Plus on one of my ARs now. It has a much brighter dot than the Aimpoints I've looked through, under the same conditions. (I've shot with a guy a couple of times who owns every optic you could put on an AR: Elcan, Aimpoint, Eotech, you name it).
I shot the rifle with the Aimpoint, then he pulled that off and put the ProPoint on...I had to turn it down, the dot was so bright.
Now, with the ProPoint, you obviously don't get the parallax free sight you do with an Aimpoint. For casual plinking, though, I'm sure you can live without it. Plus, in the old Eotech vs. Aimpoint debate, everyone creams over the 4moa Aimpoint dot for CQB. Well, the ProPoint 3 has a 10 moa dot, so there! [:D]
My only bitch with the ProPoint is the mount...the rings are cheap, and the Plus model is a 33mm tube...so nothing else works on it that I'm aware of.
I also just got an Aetec 3.8x12 scope. I paid about $135 for it, but I've heard Wally World has 'em for $100. The scope is crystal clear, very bright, and although it doesn't have target turrets, the adjustments have tactile and audible clicks that are better than ones I've seen on much more expensive scopes.
I had a Simmons Pro Hunter 6x18 on my Armalite for a year and a half, and never had a problem with it. That rifle shot so many 1/2" groups in that time that I still use it as the standard to compare all my other ARs to.
Now, I absolutely agree that there are cheap POS out there, no question. But there are also some inexpensive optics that will certainly do what you need them to, and won't break or fall apart. Finding those while avoiding the POSes is the trick. That's where experience REALLY pays off lol.