Quick and easy integration.
They react to feature requests and bug reports.
Their prices are fair. At least compared with Apple's 30%.
In the beginning, the support and the communication was a bit chaotic. But that improved a lot within the last year or two. However they still redirect every support query they get from my users to me, even though they could solve it by themselves. (Like resending a license code to the user)
Their vendor website feels a bit dated at some sections, but all in all it is usable.
The documentation isn't very detailed on some parts.
I've been one of the early users of Paddle since they started a few years ago. Their service helped me a lot to start selling my products (Software for macOS) easily without having to think about foreign tax, user and license management. It helped me a lot to grow my company. I wouldn't want to do business without the help of Paddle anymore.