So far, 157188 users have downloaded API documentation sets while using Dash, my documentation browser app. The chart below shows the unique download counts of individual docsets.
The data is collected by analysing the server logs and counting the unique IPs that have subscribed to a docset feed. As Dash is an OS X-only app, this data is just a slight indicator of what OS X developers need documentation for and nothing else.
One thing surprised me: over 60% of Dash users had a need for web development docsets at some point in time.
Note: The OS X and iOS docsets are missing from this chart because they are not hosted on my servers (Apple hosts them).