I was under the impression for quite awhile that .223 was too light for hogs and stuck to 7.62x39 only in my semi-auto rifles. I was wrong though. I got my first AR-15 a few weeks ago and decided to take it for a spin on hogs just for the hell of it. I ran up on a ~300-350 pounder, put a couple of Federal hollow points in him, and he probably got about 15 yards before dropping. Not bad at all IMO.
I will say the 7.62x39 will drop a damn big hog on the spot (assuming good shot placement of course), but that kind of damage isn't required for me. All in all I'm happy with the AR-15 in hog hunts, and will use in the future for the sake of accuracy (over my AK).