HappyFox is certainly a worthy alternative to Zendesk. Quoting Mashable.com: "Foursquare uses Zendesk, but other popular options include Get Satisfaction and the beautifully designed HappyFox." (http://mashable.com/2011/10/19/downtime-communication-tips/)
HappyFox is way simpler and we do have zendesk data import feature that will move your data too.
It is the only product which is time-tested both with large corporates and small businesses. HappyFox was originally a product I launched as Helpdesk pilot, in 2005 with in-premise and traditional licensing model.
Our email integration is simple and unlike Zendesk gives you email integration that works with your domain name right from the start. We were the first ones to launch a free plan that actually has features that you would need like Advanced Reporting and professional support.
We'll soon be launching our new pricing and a switch program that will allow you to make the most of our existing investment into other products.