I've always used the "alternative combat system" when playing OD&D, as it's the one my players and I are most familiar with, but I've often thought about giving the original Chainmail combat system a whirl in order to get a sense of what things might have been like in the earliest days of the hobby. Unfortunately, though I own a copy of the game -- sadly only an electronic one -- and have read it several times, I'll admit that I don't quite get it in places and never felt entirely comfortable making a go of it. (Yes, I realize this reveals an awful truth.)
Thankfully, D.H. Boggs, author of the speculative clone, Dragons at Dawn, has put together a nice little booklet to help mental deficients such as myself. Entitled Using Chainmail to Resolve OD&D Combats, it's available as a free PDF here. And while you're there, consider picking up a copy of Dragons at Dawn. I haven't gotten round to it myself, I confess, but I plan on doing so soon, if only to show my thanks for this excellent OD&D resource.