I took a quick look at the site and took a demo test -- its an interesting concept, and I think could be quite helpful when studying for the exam. My only complaint is that the site is pretty slow to respond between exercises. Hopefully its not as slow when you pay for the tests. Because this site charges for the exams -- I'm moving this to Blatant Advertising.
I agree with Jess - it's a nice concept, but it's painfully slow. It took me over 7 minutes to get through the 3 practice questions (and I didn't spend much time thinking about them)...not to mention I'm on a broadband connection. If you could make it run faster, I think you'd have a nice app.