On top of what others have said, the Watchface/App dev experience is pretty great. The OS provides a lot of compositing and animation features that encourage really lively and cute designs, and the Pebble app has a JS runtime that allows apps to do whatever phone-side stuff you need without having to build separate Android and iOS apps (or, as a user, install a ton of companion apps). Spin-up and iteration is really easy because pebble-tool manages building, deploying to QEMU, and running the phone-side code in Node.js so that you can launch and test your app end-to-end with one command.

Having to write C on the watch-side isn't everyone's cup of tea but they are actively working on a replacement for rocky.js so that you can write everything in JS.