Soda, not flamin' just asking for negative user experience.
Oleg, with the carry handle mount you could knock the rear sight off and still be pretty close, not obstructed at all. I did fool myself once though, my killflash was shifted and cutting off a little of the top of the circle and I didn't notice, was shooting rather high as a result. I guess I'm just a anachronism, I can't stand flattops and feel that you have to do some contortionist act to align your eyes and face to a stock with no drop. The straight stock and pistol grip put my eye up where the iron sights are. For the ACOG I back my nose off the charging handle about an inch. Somtimes I like a little spot rubberized grip tape on the stock in the manner of a bow kisser button to get you used to sticking your face in the same spot each time.
Now that I asked for negative, I'll go for why positive for me. Someone alluded you could buy a real scope for the price of an ACOG. Yep, also could buy a wild night in Vegas or something, who cares. In their quest for "real" scopes many American gunners buy way too much scope. High magnification can help you shoot tiny groups but can really impact usability. It is "too much syndrome" I came into a Rem 700 that had a fixed 32x scope. I could shoot 5 shots touching all day long, I could also read what the NRA had to say on the bottom of the target at 100yds. My field of view at 200 was maybe 10 feet. I had enough trouble getting it lined up with my target, imagine something moving! If there are people shootnig 600 and more yds on iron sights you do not "need" high magnification. You also do not need the compromise variable power scope in most situations that an AR can handle. Variables have more moving parts.
Parts that like batteries, quit.
So, I like:
-Low moving parts count
-Lack o' batteries
-Low fixed magnification. Decreases extreme precision, increases shootability. (See also less moving parts)
-very durable construction
-compact size
-simple, repeatable mount. Rock solid.
-And finally, that cute little red crosshair I can see in the dead of night.