here's some more of MHO. I have used Aimpoints every since it was issued to me in the 101st as a FORCEMOD item in 96-97. Great sight, the army alone has proven that soldiers in basic training tend to shoot better with the Aimpoint on qualification ranges than with irons. That same success has been taken to CGM/CGB ranges and fights and proven as well. I have never had any issues with the Aimpoint, it serves its purpose well and I have seen them severely beaten up and still function.
I have heard/read/talked about ACOGs for quite a while, but never purchased one. I borrowed one from a close friend before deploying to OIF (he said "take it with you"....talk about a good friend). Since I had not used the ACOG much I went in with my Aimpoint set on my rifle (M4 w/: AN/PEQ-2, RAS, ARMS#40, homemade 3-point sling). The lack of magnification on the Aimpoint severely hindered target/threat identification. A major part of a soldiers bout with Rules of Engagement. I quickly switched to the ACOG and the problem was solved. One of the major lessons learned from my standpoint and that of the soldiers was the need for an optic that has the magnification to id the target, and as newarguy point out, make that less that perfect shot on a target who is trying not to get hit.
We did have some ELCANs (very few) or as the Army calls them Machine Gun Optics (MGOs; designed as a day sight for SAW/M240B, but mount and work extremely well on the M4/M16) that we put to good use with success. The sight is bulky and does not have the benefit of Tritium to power it at night. A lot of my guys found the reticle would wash out against dark backgrounds (not so with the Tritium powered ACOG).
Not everyone will be put in that situation, but it is a combat lesson learned that I will not soon forget, and if I had the money I would buy every soldier in my company a ACOG. I probably sound like a sales guy for Trijicon, but thats not my intent. Just wanted to put my two cents in on why I think anyone who puts up the money for a ACOG will not be disappointed. And just for the record, I own an Aimpoint that still gets a lot of use at home.