I`m using the NBDE mastery for the last couple of days and I can say it is probably the best app for the boards I have the Q&A first aid app too but I like the NBDE mastery more, it is similar format to the decks and it has the mnemonics in one place so you can find them easy within seconds.
It helps a lot but you still need a some kind of reference book FA,BB or the decks JMHO.