It has gone over one and a half year since we've started selling paid subscriptions for our extensions. We've tried different Joomla components for selling paid subscriptions/downloads and finally used VirtueMart, a popular e-commerce component, for over a year.
In the course of time we've faced a lot of problems; unending bugs of VirtueMart because of it's slow development, old/hacking style code base, very hard to manage/search orders, not able to define different limit for each subscription and lack of lot of features etc. which has obligated us many times to modify the code ourself.