I finally went ahead and registered as a developer for TheEchoNest API. It’s easy to do and only takes about a minute after which you can immediately send requests.
The query format is pretty straightforward and can be sent as POST or GET (not always allowed as GET though) requests, with response such as the one below in either JSON or XML format, which can then be parsed by your code.
There’s obviously a lot of powerful applications (such as Spotify) that make use of the API, but there are also plenty of demos and tutorials for both the developer API and Remix API to get you started and to give you some ideas.
My favorite demo I’ve seen so far is the ‘You Make Me Quantized Miss Lizzy‘ post, where the API is used to correct some less than dead-on drumming, mess about with the tempo and also the pitch for some interesting results.