I'll second this recommandation. This kit became is my go-to choice for prototyping more complex microcontroller projects. Plenty of peripherals, flexible pin muxing, good documentation and the ARM core is quite fast. Finally, I almost forgot about the open source HW debugger to GDB bridge[0].

I actually wrote a set of drivers, libraries and a build system to allow easy prototyping (like Arduino) but also enable access to the more complex features of the SoC.

[0] https://github.com/texane/stlink