Tom, like gsganzer, my personal all round preference is the IR231; quite possibly the most popular 1/2" impact wrench there is, and what I used for my personal impact. However, Chicago Pneumatic's CP734 probably runs a close second among mechanics and was my brother's favorite primarily because it's so easy to reverse while operating with one hand. There are probably more of those two in use by professional mechanics than all others combined and they're both rated with the same power. /forums/images/graemlins/wink.gif However, the more recent IR2131 is even more powerful, and easier to disassemble if you ever do need to repair it, and gained a lot of favor with diesel mechanics.
Keep in mind that you're now talking about my personal opinion after working on and using a lot of different brands and models, and also that I got out of the business a couple of years ago and some new products have come on the market since that time that I'm not really familiar with. One I can think of is a newer model Chicago Pneumatic that is a clone of the IR231; don't know if it's as good, better, or the same.