FastSpring is very easy to setup. We integrated it to our products in a day and our business was running in a couple of days. Easy API integration, which allows us to keep records of customers, orders, etc is probably the best feature for us. We are also very fond of FastSpring since it enabled us to offer many payment methods to our customer base.
Moreover, FastSpring offers many other options for your e-commerce. You may offer a subscription based service or product to your customers. FastSpring handles the auto-payment and subscription processes smoothly. It is also possible to track the subscriptions customers privately, by using API integration. For any product, you may also setup volume pricing, discounts, coupon codes, etc.
We haven't come across any major problems with FastSpring and when we do have a problem, the support team helped us through it.
On the other hand, I would say that setting up a store page for your products is not that trivial if you are looking for a custom design. Especially designing a payment site that looks similar to your own web page is somewhat difficult.