Over the past several months, I've built a business off of buying items on eBay and reselling them at a higher price. It should be obvious why a sniping service is critical to the success of my business - and Gixen is the only one I trust with my bids.
For the first few months, I used the free version and this worked great, but I went ahead and upgraded to the mirror service to help support the service and because a couple of the additional features apealed to me.
As things are now, the only reason I can imagine changing services is if I were to develop my own custom sniping service for personal use. Other than that, Gixen is going to continue as a large part of my business.