While I had UWorld for a year subscription, I will say the questions are not always sufficient in preparing for shelf exams on certain rotations. For example, there are only 100 or so Psych questions, 200 for OB, 300 for Peds. So obviously the QBank is more beneficial for things like IM (>1000 questions).
I supplemented with other question books such as Pre-Test for Peds and Lange Q&A for Psych.