This is one of the simplest cart systems I have ever used. It is also very aesthetically pleasing right out of the box.
This system could also be used for 'hard goods' as well - it does not have to be limited to just digital downloads.
I had a couple of issues, but within 6 hours of submitting a ticket the developers had provided the solutions. They even tweaks a couple things that was not their issues - Awesome Support!!
I would highly recommend this extension!!