Ok, so it's clearly not going to replace any other QBank, but that's a bit harsh.
Since I posted this, I've done 600 questions already. Since nobody else seems to have much info, I'm going to post my opinion so far.
Of those 600, 2 had a spelling error and 1 had a questionable answer (i.e. two answers were equally reasonable). 95% have very good explanations, 5% of the explanations suck. The question difficulty can vary a bit, and it's clear the qbank was written by many different authors.
EM has a great categorization system, and it allows you to easily see which subjects and systems you're weakest in. It seems pretty accurate as well.
As for subjects, their genetics and statistics questions are well outside the scope of anything my school teaches. They also throw in some clinical questions on par with Step II. (e.g. psychiatry, pediatrics, IM, surgery questions - they are identified separately in the question breakdown) Fortunately, those are rare. I'd have to say that the questions were almost certainly written by very knowledgeable authors.
It's also a great tool for learning. Not only are the explanations good, but question stems contain prevalence information and realistic case presentations.
So if nothing else, I figure by the time I get through all 10,000 questions, I will be in a good position to begin studying for the boards.
Overall, I won't say that it sucks, but I can't say that it's particularly great - just like my medical school.