Heh, how far can you see? That's the maximum effective range. The only factor that can tamper with that is how large the dot is. Remember, a 4moa dot is 8moa @ 200 yards, and 12moa @ 300 and so on...
How far away can you reliably hit someone when most of their torso is covered by the dot, or is totally covered by it?
Figure this. The dot is only 4moa when it is on medium or low settings. If you have it blasting on 9-10, the corona effect makes it larger.
There's the 2moa version. That should theoretically help at longer ranges, but I feel the 4moa is quicker to pick up, and that far outweighs the ability to pinpoint a spot at 600 yards.
In my opinion, the Aimpoint is best from 25-200 yards. That is the sweet spot. I can easily keep the dot on a man size target at that range. EOtech reticle is probably the best for under 25 yards. That big circle is great for the in-your-face type situation.