Reviving this topic with a couple of news items:
- Les Pounder did a lovely blog post about these boards, making a micro weather station.
- a number of the issues with the C3 are now in better shape (WiFi is no longer blocking the serial in Arduino, and the RMT stuff has all gone away in latest MicroPython)
- I've just tried a custom MicroPython board definition. The helper module can be improved, but I am pretty happy with the fact that I can roll my own builds now.