Well, from what I gather..
The ACD is already calibrated out of the box. So you get "true" color. Kinda like if you bought a high end TV , the place that sold it will offer to calibrate it for you. (for a cost) Same thing with monitors, if you go to your local best buy and look at there monitor wall, they are all different colors when your looking at the display, the colors are not the same. I guess with a ACD if you lined them up like that they would all look the same.
Then you have refresh rates... Yes other monitors have higher refresh rates, but the ACD is all about displaying color correctly. So, if your a graphics pro, ACD is the way to go. If your blowing stuff up in video games, another monitor may serve you better.