You miss out on half the fun if you use software to do your sniping. There's nothing like sitting in front of your screen with a countdown timer, waiting to hit Enter just as the auction ends. Wondering if you'll get the timing right, whether your bid was good, and even if you just typed your password in properly, really gets the heart racing.
Also, a person can snipe better than these services. I can consistently bid in the final second of an auction. Bidding 2 or 3 seconds before the end is taking it easy. Services like Auction Sniper recommend bidding at least 5 seconds before the end.
But if you're away from a PC when the auction ends, they are the way to go, natch.