One of the resistors apparently need changing for USB to work and apparently you cant connect external power and usb at the same time.
http://wiki.stm32duino.com/index.php?title=Blue_Pill#Hardware_installation
You also need at USB to serial converter to get started with it (cheap from china).
I have only uploaded the blink sketch but will be trying to implement more when I have time, my only concern is that library support is going to be more limited than typical Arduinos (but im new to it, so I can't tell you it is good or bad), but if i need to do something really basic at higher speed than Arduino it could be very handy and affordable.