Why? Because it's SUPER small, reliable, affordable and adorable. The only downside I could discover was the color green. But teensy-creator Paul Stoffenregen recently added a PURPLE one to the repertoire. In collaboration with OSH Park
The tiny board runs a full 32 bit ARM processor which is a similar architecture to what is used in many mobile phones today. It's using an adapted Arduino language called 'Teensieduino' - a software add-on for the Arduino, to run sketches on the Teensy and Teensy++. It is also possible to program this board directly with C or still use the Arduino IDE with just a small patch. Teensy 2.0 runs 5V, teensy 3.0 runs 3.3V and the newly introduced Teensy 3.1 ($19,80) runs both 5V tolerance on digital outputs as still running on 3.3V as well. Today most new chips use 3.3V signals, but many legacy products output 5 volt digital signals this was a little bit switching back and forth between Teensy 2.0 and 3.0 prior.
Also - on the Teensy 3.1 the RAM has quadrupled, from 16K to 64K. While 16K is plenty for nearly all Arduino libraries, 64K allows for more advanced applications. Icons and graphics for color displays and audio effects requiring delays, like reverb and chorus, will become possible on Teensy 3.1. Flash memory has doubled, to 256K, and provides double the memory bandwidth.
A perfect controller for more advanced projects that can make use of the power available on-board.
Teensy website: https://www.pjrc.com/teensy/index.html