Had an RCO on my A4. The ACOG was one of my favorite pieces of issued gear, and was one of the only pieces of gear that seemed to be completely Marine-proof. Once a boot left his rifle by the porta-potty when we were out on an FX. A young NCO found it and promptly disassembled the rifle, including taking the RCO off of it, and buried it all in the California desert as a valuable lesson to the boot Marine, who spent the next two hours looking for his rifle, piece by piece. Finally it was getting to be our turn to head out to do some night fire with our PEQ-2s and he still hadn't found the RCO. The 1st Sgt found out and was pissed, not only at the boot Marine, but at the young Cpl too. An entire platoon ended up shoulder to shoulder on their hands and knees combing the countryside for that RCO until we found it. Being buried in the sand for several hours didn't seem to affect the optic any more than being banged around in an APC or anything else we did to it. I never saw an RCO get broke or even lose zero.
The RCO is the 4x version of the ACOG the Marines put on pretty much every rifle in their inventory. One Commandent or another called it the greatest increase in the effectiveness of a Marine since the adoption of the M1 Garand.
It is an excellent optic for medium range, but once you get under 100 yards or so, I think you'll find that a red dot is faster and easier to use, esp on the move or from awkward positions. The RCO on our issue guns had a very short eye relief, which cause the ocular lens to nail you in the eye brow when firing from awkward positions, which happened alot since the M16 doesn't have a collapsable stock. If most of what you're doing is at bad breath distance, their are better choices.