I have the great circle apps, which are mostly good apart from the app itself crashing halfway through a test, at least once per day (on iphone anyway).
I don't like that you have to do random questions every time, and not being able to pick a particular topic of a subject to work on. All things that can be improved with it.
Although with that said, for the price of it and some of the questions which came up in my exam (which were nowhere to be found in some of the books) these were very helpful, any may have contributed to those few extra marks which make the difference between an exam pass or fail.
Overall - recommended if you can put up with it crashing now and again.