I purchased the Ability Mail Server 2 and the Ability FTP Server after I signed my first client back in 2004(I think that's when it happened). I currently host about 30 sites with mediatemple.net. Although they have extremely stable service, I have always run a local Windows box with a mirror image of what is on my live servers. This includes using the FTP server to copy all files on my live server and using AMS to receive and send mail if and when the live server goes down.
I have used AMS off and on for the past couple years for only a handful of accounts. Both times I have contacted Code-Crafters about my lost/forgot keys or installation issues, they responded faster than I ever expected. If you are looking for a great product that a company will stand behind, you have find the right product.