Ok, so I don't really use the DOF preview button much, because I am A) shooting wide open, so what you see is what you get, B) stop down a bit and am not too critical of the DOF as in most of these cases I can just recompose if need be. I just realized that in the custom button function (accessed either through the menu or the quick menu) you can set the DOF button (which is in perfect position for quick access) to engage AI servo when you are in single shot mode. This make AI servo soooo much more accessible because you can turn it on at any point, if you subject begins moving, or if you have a moving subject on occasion, but you like the focus confirm of single shot most of the time. you do have to hold it to keep AI servo on, but that is even better because you can dynamically turn it on and off with the tip of your ring finger.
Also, I'm kinda bugged by people complaining about the speed of the AF in low light and missing potential shots. If you want less AF accuracy and more shutter release speed just change the third AF menu box "one-shot AF release prior." more toward shutter. The default is to max AF accuracy priority. Sorry for the rant. I have seen a TON of complaints about minor things that can be easily changed in the menus (I'm looking at you ken rockwell, "this ugly AF box is always getting in the way of my cat pictures" and "I am too busy fiddling with the AF mode to get a picture of my grandkid", learn to use the M-fn button with your pointer finger and the AF select button with your thumb).