Our tests are mostly based on Arduino Duemilanove model (Duemilanove is Italian pronunciation of 2009).
Reason is simple: it's cheap, much diffused, highly supported by libraries, reach of free utilization examples and the last produced platform where the microcontroller is in DIP (Dual In-line Package) and not in SMT or SMD (Surface Mount Technology/Device), so changing a controller with a new one is immediate just like mounting it on a breadboard.
Arduino 2009 has the ATMega328 microcontroller on it: as you can read on datasheets (Summary and Complete) it is a 8-bit controller with 32KByte of flash memory.
More than enough for multiple use cases.
Arduino isn't the only solution available, but interfacing between Hardware and Software becomes easier with a so powerful programmable platform.
So no excuses are worth much!