Trijicon ACOG is a a pretty darn nice optic. Onto the usual format.
The Good: These things are rugged and reliable. They have a day/ night illuminated reticule that doesn't require batteries. They are really easy to mount on a flat top rail. If you remember where you mounted them they hold a zero well enough for functional use. Between the generous eye relief, wide field of view and Bindon Aiming Concept they are easy to shoot well from very close to a few hundred meters. Their bullet drop compensator is very quick and useful after a little bit (like a single trip to the range) of practice. With a properly zeroed ACOG most people shoot better than before. I know I do.
It is my opinion that the ACOG is the best all around combat optic I have used or seen to date. I could go on and on but it is easier to say they are really good scopes.
The Bad: They are expensive. These are a lot nicer than Chevy grade optics costing less than half as much, as they should be. An ACOG can easily costs as much, if not more than many AR style rifles you could mount it on. Depending on the exact model you want they start a bit above a grand and go up from there.
The Ugly: The only real ugly is just a continuation of the bad. They're just too expensive for many to willing to pay for or able to afford. I can certainly understand that perspective and don't think you should go out on a financial limb to get one. Also I get just being a cheapskate. The high price point and the resulting sticker shock of the ACOG is such that they aren't even considered as an option by lots of folks.
Bottom Line: If you can deal with the sticker shock and hit to your wallet I strongly suggest the ACOG. You will really like it.