I find that developer documentation is often not very pleasant to look at and, more importantly, often very difficult to navigate. I worked briefly with the PayPal REST API and, while I found that at times it can be confusing to deal with the numerous APIs PayPal offers, aesthetically their developer documentation is clear and effective.
What I Like
Clear Overall Design
Overall the documentation looks nice and clear. I like the fonts and colours used. The multi-level menu on the left fits in well and provides good means of navigation without feeling overwhelming.
Good Instructions and Code Examples
The subheadings are easy to spot and the step-by-step instructions weave code into them quite nicely. Code examples stand out due to the different background colour.
Clear and Informative Footer
The footer for the page fits nicely into the overall design and gives access to a wide range of resources.