I like using the Adafruit Trinket M0 module in my projects but wanted a little more flexibility. I wanted a removable Sam D21 chip holder, onboard breadboarding capabilities, swappable voltage regulator and in circuit test points for accurate current draw measurements. Since programming is done in circuit-python and Arduino I also included interconnect for a Atmel Programmer to flash the bootloader. This has made for some nice custom work around the Sam D21.