Cheese,
I have gone through most of the available optics on the various AR's I own or have owned, so I'll give you my opinion. The Trijicon ACOG is the Cadillac of combat sights, quite possibly the best you can buy. They are remarkably dependable, overbuilt, and reliable in virtually any situation or condition you may find yourself in. I had a TA-31F on an M4gery and loved it. Depending on the reticle, it is easy to make hits at 25-300+ meters. Glass is great on these scopes, and the dual illumination allows you to use the sight day or night. In terms of toughness, I would place the ACOG against any other optic available.
However, I did find a few disadvantages to the ACOG. First is the price. The ACOG is not a cheap weapon sight, and at $900 becomes unaffordable for a lot of shooters. Also, the ACOG has a relatively short eye-relief. All though this isn't an issue on light recoiling weapons, it still does take some time to acclimate to. I had to really get my eye close to the bell, and never really felt comfortable with this while shooting. Another issue I had was with true close-quarters shooting, mainly under 25 meters. The 4x magnification makes quick hits a little more difficult for me, especially with the shorter eye relief. The last negative, if you can call it that, is the reticle is very bright in direct sunlight. You can alleviate this by covering the fiber optic cables, but you may not always have time to do this.
Now, don't get me wrong and think I don't like the ACOG based on these negatives. I just want to provide you with my experience. I would buy another one in a second for the right price. If you do buy one and decide you don't like it, there is a long line of folks ready to buy them and resale is very good.
