I've been using auctionsniper.com for years and I don't even place normal bids anymore... if it's not buy it now I'm sniping.
I lose about half the time and a win about half the time but I never overpay and never have to camp out for the end of the auction
auctionsniper's price seems a bit higher.. you get 3 free snipes to start and after that they charge 1% of the item value if you win... so $1 for a $100 item with a minimum of 25 cents and a max of $10.
One feature I LOVE about Auction sniper is bid groups. Basically you put a bunch of items in a group and it will auto snipe every one of them until lit wins one and then it stops.
So for example say you wanted to buy a Game and there are 15 auctions that all end at different times. You want to get it for a really low price so you setup all 15 auctions as snipes and put them into a bid group.
Auction sniper will snipe the first ending auction and if it loses, then it tries to snipe the 2nd and 3rd and then 4, etc.
as soon as it wins an auction in a group it stops sniping.
I did this when the Wii was first released and was going on eBay for twice retail, I managed to snip one for only $20 over retail using bid groups... there were so many auctions on eBay that I managed to snipe one in the middle of the night that most other people happened to miss.
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Given the price difference Gixen sounds like a better deal, but I have no idea if they offer bid groups like AuctionSniper.