I like that everything just works. The control panel is very fast and response. I am using Fastspring's subscription features for our OCR API platform, and its regular, classic shopping cart features for the web automation and desktop automation software we sell. The setup has been a breeze in both cases - and since then it just works.
We are using FastSpring for more then two years now, and there has been no downtime or other technical issues.
The few times I had to contact Fastspring tech support with a question, the answers were fast (same day) and useful.