Gixen has been totally reliable, easy to use and works on foreign and local eBay sites, which is a huge advantage. I live in Australia but often purchase on ebay USA and German local sites.
I used to use AuctionSentry but the constant updates were a pain especially with a few bugs for non-USA customers. I've never had any issues whatsoever with Gixen, and because it's online, it means I've never missed out on an auction with a dropout or computer issues.
I'm a huge fan. I purchased the mirror update for $6 - it's outstanding, so reliable. Can't get better value for money - I don't use anything else to set bids. Highly recommended and well done to the developer - it's brilliant. Wish I could give it 6 stars!