Specifically I want to point out their Sandbox program. That's a great tool to demo the whole payment processing without actually spending anyone's money. You can set yourself up as a merchant, setup demo customer accounts (and give them virtual money, yes that past was fun!) etc.
Two topics for improvement, I thought, were:
- Documentation(of course) could be arranged much better with description of the APIs and programs, code samples, perhaps ready-made generated code etc. specifically, there are contradictions in the current documentation.
- Arrangement of the Sandbox and developer site: arrange the program and materials for each program, for example, or a step-by-step tutorial wizard that takes you and shows you what you need. For example (just to show how short-attention I am) took me ages to figure out there's that PayPalBase dll that provides the required functionality that you need.