About USMLExchange. . . just downloaded the lite version and did 10 quick q's. My impression was that it was super easy (compared to my other q-bank. . . maybe not representative of the exam) and a little buggy (can't scroll down to question review on the review screen. Any issues with the full version? $50 is pretty cheap for an on-the-go 5000 question bank. . .
IMO this is a relatively under-utilized market to this point. Wish there were more options out there!