I use BidSlammer.com for sniping auctions on the rare occasions which I place bids.
By bidding in the last seconds of the auction, I have eliminated the seller's opportunity to shill the price up, I've not indicated my interest in the item to other sellers and I've not allowed myself to get sucked into the emotional aspects of bidding - and bidding wars.
With the sniping service, I just put in the highest price that I would be happy to win the auction at.
I don't end up getting into competitions with other bidders and paying more for an item than it's actually worth to me.
I usually win, but even when I don't, I know that another widget just like it (or better) will be posted for sale at some time or other, so I just wait. :)