Ok, I have done Kaplan, UWorld, and now I am using Rx. So, here's my take on Rx.
It is not a technically clean an operation as UWorld. The explanations are not as good as Kaplan's.
But as far as being challenging, I disagree with what the above posters were saying--Rx has some pretty challenging questions, just don't check the box for "Easy" questions when you are making your tests. There are still well over 2k medium and hard questions, and they are as multilayered as UWorld's. Now, of course, if you add "Easy" questions to your stuff you will inflate your scores.
They also seem to have straightened out the technical issues I had been having.
So, all in all, it seems like a good QBank to do either with coursework, or after doing UWorld.