The M5StickC is a lot of fun, and it is relatively easy to use. I am only giving 4 stars because some of the documentation is hard to decipher, and because the ENV hat is about 5 degrees C off.
UI flow is really approachable, and fun for a while--you can make things happen fast--but as a programmer, all that mouse clicking just got in the way of what I wanted to do.
Micropython was cool, and worked pretty well, but if you want to be able to use and adapt other people's libraries, learn from examples, Arduino is really the only way to go.
I'm still trying to wrap my head around the accelerometer & gyro maths and 3D, though!