I like FastSpring and used to use it with Poker Copilot. However I stopped due to FastSpring not letting me denominate my account in euros. As most of my customers are in the euro zone, and I'm in the euro zone, having to change funds from EUR -> USD -> EUR was a major leak in my finances. (I wouldn't advise most people to worry about such a thing, but as my product is mature and sales are not really growing that much, it was worth my time fixing such leaks.)
I changed to Stripe (which denominates my account in EUR), implemented the EU VAT stuff myself (not a good idea, I know, I know...), and now net significantly more money.
Another advantage over Stripe compared to FastSpring is that as I could hand-roll my own payment page, I could get rid of a lot of the extraneous fields FastSpring has as part of a generic solution. The simpler the payment form, the more sales completed, goes the logic.