I use the Teensy 2.0 board. This one:
http://www.pjrc.com/store/teensy.html(because its nearly an arduino board, it is in fact arduino compatible)
i like it because:
- programmable with plain c
- programmable over usb
- small board
- cheap board
- provided usb library works and is simple
So if you decide to code your arduino thing in c, I can help you. If you use the sketches, I can't help you

My projects so far with this board:
- controller for a 'frets on fire'-guitar. Haven't made the guitar so far, but the controller works

- ad-sampling with 7.8kHz, sending the data over usb-hid and on the computer i make an fft (result is ofc a spectrum analyser for audio below 3.7 kHz (niquist))