It is not Arduino compatible in the meaning of taking shields (just like every small form factor board isn't).
But it has all the same pins (exept 3.3V power and Vin*) available as female headers. The input and output pins that are right next to each other on original Arduino, almost are the same here, except digital 4 and 5. That means the order is:
RESET 0 1 2 3 4 VCC GND _ _ 5 6 7 8 9 10 11 12 13 VCC AREF GND A0 A1 A2 A3 A4 A5
It has power indication LED. Not an essential part, but it reminds you to remove the battery when not in use and I have also found it to be useful to notice shorts in my circuit – the power LED goes dim in that case.
It does not have 13th pin LED.
* - Mine has only 9V battery clip directly soldered to it, but this is the easiest part to modify. It also does not have ICSP header, but in the rare case a ISP programmer would be needed it is possible to access all these pins through female headers or remove the Atmega from socket.
So what's the purpose of such thing? I felt I needed a secondary controller besides my usual Arduino for testing things out. I missed that when programming my robot for a contest where IR transmitters were supposed to indicate important places and my attempt to make ATtinys do similar transmission failed. And now I have a task of learning about radio modules ahead of me and I want a familiar platform to test them out before trying to also mess with microcontrollers new to me that are supposed to go into the final product.
When considering the price remember these aspects:
- Do you already have FTDI cable or for how much can you purchase it? (I managed to make one myself, but it takes precise etching and SMD soldering. Also the needed chip is not super cheap for experimentation. Further details https://www.instructables.com/id/FTDI-cable/ )
- Do you have an Arduino (in working order) or AVR programmer to use for burning bootloader on a blank ATmega or can you get the controller with bootloader for reasonable price?
If all that described suits you and you feel ready to do some tightly packed soldering – it really wasn't too easy to get all wires to their right spot – then here is my documentation of the project.