I thought Crush was horrendously superficial, suited more for a primer than for an actual book to study from. If you want to refresh yourself of everything you learned in 3rd year of medical school, and want an easy book to read in a weekend, Crush is it.
I did use First Aid. People recoil from the idea because it is very text heavy. Its a long winded read, requiring you to annotate rather than expand. People who used First Aid for Step 1 were used to seeing loose associations of words getting them questions right, adding explanations manually when they got a practice question wrong. In Step 2, you will find yourself editing out the excess crap they included for completeness. Completeness gets you more points and a higher score.
Step-Up has a great layout and good diagrams. I felt that it was NOT the best choice, as it fails to cover everything on the test. It covers most things (focusing on medicine). To complete you education you need the series, which for me is too much.
Kaplan Qbank + First Aid + UWorld = enough for me.
