I had this exact same question a few years ago, and I was looking at dropping a good deal of cash on the ACOG (which, by all accounts is a superb product used by our own special ops units).
So I took it one step further to e-mail some of the folks at places like Gunsite to see what they had to say. Their answer was something I never considered: TASCO. I know, I know, TASCO = cheap, right? Well, maybe. However, since CDNN was (still is) selling the TASCO PDP5 for $80, I thought maybe it was worth a look. So I mounted one on a Calfiornia Competitionworks 2-level mount for the AR-15 (a nifty $110 contraption from Brownells catalog that allows a red-dot sight to be mounted forward of the carry handle).
I love it! Yes, I did accidentally leave the thing on and run down the battery, but the batteries are $2 each, so I have a whole stack of them in the case now. I really like the different brightness settings, and the fact that I can adjust the dot size for different situations.
It is a very nice, inexpensive alternative that you may want to look into.