Great stuff, the iTunes copy works really well for me, a pain to update perhaps but I'm at the stage where I buy an album fairly infrequently so it's not too onerous.
I went for a tiny Sandisk Cruzer 64gb, which is impossible to see from the outside (and even when sat in the car if you don't know it is there!). The odd red light for reading when you unlock the car but again, you'd think it was an alarm if you didn't know.