I really can't say enough good things about Gixen.
The basic service, which is FREE, is terrific. The upgrade, which is nearly free ($6/year at this writing, and no, that's not a typo, I really DO mean 50 cents a month) offers a mirrored-server service, pretty much guaranteeing fault tolerance.
The only times I've heard of Gixen failing have been when eBay changes stuff without notice, which is going to cause problems with ALL automated sniping... and Mario (Gixen owner/admin) copes with even those problems far faster and more effectively than any reports I've heard about other sniping software or web services.
Mario is also blazingly fast to respond to support questions on the Gixen user forum, and the one time I had reason to email him, he likewise replied immediately.
I live out in the boonies and have a slow/unreliable Internet connection, so Gixen offers me not only "set it and forget it" convenience, but also a far more reliable way of placing bids than if I tried to do them manually.
I have (usually unrealistically) high expectations of services, software, and products in general, and very, VERY seldom run across something I absolutely rave about -- I'd say Gixen is one of 3 such items I've encountered in the last, oh, 10 years or so, LOL!