I've been using eBay for many years now and got so frustrated being outbid at the last few seconds of an auction - even more frustrated having to stay awake in to the early hours to put in a final bid on something I wanted!
I tried other sniping tools such as Bidnapper, but Gixen wins hands down. The free service works fine for me, but I will be paying for the mirror service which allows biding right up to a couple of seconds before auction end (at only $6/yr it's small beer and feel I need to pay back a little for the money I've saved over the past few years),
The Gixen service is simple to use and enables me to stealth bid on and group several items without having to monitor the bid process in ebay. As with any sniping tool, you should expect to be outbid regularly and not every bid will be successfully submitted, but that's all part of the fun of the chase.
The only criticism I would have of the product is that it doesn't retain a history of your bids - successful or otherwise.
Nonetheless, a great tool that I find indispensable for my ebay purchasing and which I wholeheartedly recommend.