I found the Qbank questions to be pretty representative of the real test questions.
I was getting mid-80%s on Qbank right before the test (I started out in the 50s), and I got a 250 on step I. As far as how the difficulty compares, it's quite hard to say. You'll hear people say competing things. Many people think the "real thing" is easier than Qbank. I, on the other hand, thought the "real thing" was much, MUCH harder. But remember that not everybody takes the same test...some tests are harder than others, and then the score is calibrated against the difficulty of the test. So it will vary. I think the general consensus, though, is that if you're scoring in the 70s, you should do average to very well on the real thing.
Good luck!