There are several videos on Youtube about them.
Beyond being faster the Bluepill has an RTC that alone makes them inherently cheaper.
there is an stm32duino comunity - aparently you can use the Arduino IDE and most Arduino libraries with the BluePill.
There is a Fritzing part for the BluePill somewhere, but it has a bunch of problems - if you search the fritzing forums there is discussions about fixing them.
I jumped at the STM32 for other reasons - I worked with STM32F103’s for about 5 years, so I am much more familiar with them than AVR’s.
Every CPU has its quirks and the STM32 is no different. But it is the easiest to use CPU of this power I have ever worked with.
BTW If you want even more power Several people have upgraded BluePills to STM32F3 and STM32F4 processors - Much Much more flash and RAM.
There is usually a version of each STM32 Family that is pin compatible with its predecessors.
And if you want a quantum leap more power than the stm32 - look at the OrangePi Zero.
a $6 Raspberry Pi like board that runs 1.2Ghz and boots linux.
And you can toggle GPIO pins from shell scripts.