Gixen is a great free and premium eBay sniper that allows you to place an eBay bid in the very last moment so you aren't creating a bidding war with someone else and pushing the price up.
I've been using the free service for years and I can say it has worked perfectly for me with no problems. I like that they allow for bidding groups with the free service, which means that if you don't win one auction Gixen will try to snipe the next auction in the group until you have won the item. This means that you can set an upper limit on what you want to pay for an item and stick to it without worrying about having to manually be there to bid and manage your budget for an item.
The only thing I had misgivings about at first was giving my eBay username and password to Gixen to login to the service. At first I did not want to do this but I appreciate that they need this info in order to snipe auctions and there is no 'sign up' procedure to Gixen. If you are worried about this I can say that I have never had a problem with them having this information since 2010.
I have not had a need to use the premium Gixen service but from what I understand it provides some redundancy on a snipe and offers more features for bidding on grouped auctions. The redundancy comes in the form of mirrored servers so that if the free server goes down for some reason the mirrored server still snipes the auction. All I know about the extra grouped auction feature is that it allows you to either cancel the rest of the group if the first auction is lost, or set the number of items you want to win from the group.
If there was a once in a lifetime item I wanted, or I needed to complete a set of separate items, or I was a collector of rare items there is no doubt in my mind I would pay for the premium service.
If you haven't tried Gixen I would really recommend it: 5/5 stars!