Fastspring is great, I've used them for the last 5 years. They handle VAT as well, one of the main reasons I use them (I'd rather not deal with that bureaucracy myself).

It's still a good idea to have a second payment processor, in case one has downtime or a customer has a problem charging their card. I've never had problems with Fastspring, but I occasionally had problems with others - I have a switch in my backend where I can flip to a second payment processor instantly.

Yep, we (Breeze, http://www.breeze.pm) have used it for over 3 years without any issues.