I have one, and I've only been toying around with it, but I've been fairly impressed. The hardware is very nice - polished, while still keeping everything accessible. The software is pretty decent also. The documentation leaves something to be desired.

That said I don't see it as completely overlapping with the Pi. The ESP32 in the M5Stack is a much less powerful system in terms of performance, memory, storage, etc. It's not running Linux, so your code is much closer to the metal. That can be both good and bad depending on what your goal is: you have much more control over the hardware but it limits your choice of programing languages and what existing software you can depend on.

I actually just bought one of these! I'm working on a project where I need a battery, IMU, speaker, and quite a bit of processing power (for real time audio), and their m5 core2 aws IoT product met all those requirements for only 40ish dollars.

I only just got it, but it took me only a couple straightforward hours to set up the dev environment and compile and flash their sample hello world project. (I haven't done anything with esp32 before).

I don't think it's analogous to the rpi, though (which I haven't used) --- you can't put Linux on the m5stack products because they're meant for much simpler tasks.

Not a fan of hijacking scrolling on web pages in general, but they really nailed that stacking animation!