Sounds interesting. I haven't downloaded it to test it yet because I feel like your description needs more detail to entice people to download it. Here are some questions I think a potential user (I'm pretending to be me a few years ago when I was studying for the PGRE) might have before deciding it's worth the time to download this app:
1. Does this app provide answers at all?
2. Does this app provide detailed step-by-step solutions, or just the answer?
3. How are the solutions provided -- is it like the ones you might find at the back of a textbook, i.e. not very much pedagogy/explanation, just a bunch of equations, or does it explain not just the math but how to think about / approach each problem too?
To me, without knowing these things, I probably would not spend the time to download the app and it would be fairly easy to update your description and provide screenshots that would answer this question!
While it's certainly possible to have everything on mobile devices, how useful would this be? Personally (and everyone is different!) when I was studying for the PGRE, I was doing practice problems in situations where my computer is available. If I am in a situation where I have my phone but not my computer, e.g. on a bus or something, I probably would not be able to concentrate enough to do practice problems, nor would I be able to write out equations etc. without some paper and desk space.
However, something like this could be really useful if it had worked-out detailed solutions. In undergrad, my bus commute was over an hour each way, so it might be helpful to spend that time looking up previous PGRE questions and their solutions (but not actually attempt to solve them) just to memorize different solving techniques etc.
Finally, what about the latest published practice test (from 2008)?