The G36, IMHO, is a nice gun. It is probably a teeny bit better than the M16 as issued by the U.S. armed forces. However, I don't think anybody thinks it is so much better that the Military should replace their existing weapons without the G36 being seriously revised to reflect American military idiosyncrasies. There was, at the beginning of the wars, a big clamor for a new weapon. The XM8, which is basically a restyled G36, was seriously considered as a replacement. A huge number of factors, political and practical, led the military to scrap the project. Frankly, the military seems to be doing that a lot lately. Recently, they looked into replacing the Beretta. They made a big stink, manufacturers started submitting weapons (HK45, SIG 220 Combat, etc) and then they changed their minds. The military doesn't know what they want.
Don't blame me, I voted for Cthulhu