It is indeed a great tool of this problem generator - but is it available via api calls?
This is a fantastic idea.
Just keep working at it, improving it, and keep adding more topics which range all the way to tertiary level. I've been looking for an elegant way to practice integration, series, limits, linear algebra, etc. without repeating text-book questions by rote for a long while. This will help keep things fresh and interesting.
As a tangential suggestion, I recommend extending your billing for Pro subscribers to multiple currencies so we aren't hit with international transaction fees and varying monthly or yearly costs depending on the exchange rate at the time, as well as gradually working on the engine so that we come across those "no step-by-step solution available" occurrences less often. They always make me sad. Otherwise,
I love having this feature but I want to point out some problems. These of course could be user issues. I use Firefox 35.0.1 on Ubuntu 14.10.
Integration by parts is barely working most of the time. It is very difficult to input and answer that Wolfram will either parse correctly or accept. Even if I copy and paste answers from Wolfram itself. Example: Integral (3/4)xcos(5x)
Answer from main Wolfram page: (3/100)(cos(5x)+5xsin(5x))+C
This answer is marked as wrong.
The "Input Interpretation" can be flaky. One time it worsk, next time it shows nothing. This is true of other problem generator pages.
If you check at the solution on the problem generation page. It gives a differently factored answer.
I would like to see access to the problem generator be part of owning Mathematica rather than having to pay for it via wolfram alpha.
It would also be nice to be able to have a practice sheet generated that pulls from multiple topics, rather than just a single topic.