We used inverseflow a few years ago. We've changed to Kayako as its able to handle the amount of requests coming in that a large client base creates. Inverseflow is fine until you get to a certain amount of tickets coming in. Then it slows down quite a bit. I'd personally recommend Kayako. We've been very happy with them and the helpdesk has helped us reduce our ticket response times. We keep realtime stats of them each day on our ticket submission page.

http://esupport.acenet-inc.net/?_a=tickets&_m=submit