I'd have to say that my experience with Fastspring thus far has been hugely positive. Their site is professional, and I was assigned a rep. that has taken really good care of my needs (and even went above and beyond the call of duty a couple of times).
For some background, I narrowed things down between BMTMicro and Fastspring, and actually set up both. BMT also had very good service, and I would have considered them more if I had been able to access their website (for some reason, I can't access most of their websites from my Comcast account - although other service providers don't seem to have that problem).
I really like Fastspring's account management interface (VERY slick). Lot's of reports are available and their website is easy to navigate and it's equally as easy to setup new products.
About the only things I've found that I haven't liked are the lack of a built-in affiliate program (they interface well with others, but I would have preferred a built-in program like BMT has), and the lack of pre-accounting reports (so I can see expected deposit dates and amounts, as well as breakdowns of percentages). Although I've noticed that Fastspring has been very straight forward with their 8.9% cut, and I haven't seen any "surprise" fees, so the pre-accounting reports may not be needed.
Hope that helps. If you have some specific questions, let me know.