I've shot rifles with them, and they are fine. I definitely like the principle of keeping your rifle as light and simple as possible, and only using rails when and where you need them. A free float tube like a MI-SS or Troy TRX Extreme is a more desirable option, when it comes to accuracy and versatility of mounting accessories in different places, but for the price those MOE's are pretty hard to beat.
I am ordering a PSA 20" standard upper pretty soon, and I've pretty well decided on ordering it with the MOE handguards. It will do everything I need it to, i.e. attach my sling and light, and I can always save and upgrade to a free float sometime down the road if I feel so inclined.
BTW, Onmilo, why do you have your rear sight mounted in front of your ACOG?! It seems like that would possibly obscure you vision through the scope, and would make it a lot harder to get sight picture with the irons having the rear sight that far forward.