Bottom Line: Qbooks/Qbanks are a must, PreTest falls into the mediocre category. They are harder and better than the questions at the end of BRS or RR chapters, but are more factoid and easier than the questions on shelves and boards.
For my first year classes, I used them to study for the shelves. My first year questions were far from the PreTest book, but they closely resembled my first year shelves, and helped alot.
By the time I started second year, I used Robbins Review, Lange Qbook, and Kaplan Qbank. I didn't have time for Pretest.
In the clinical years, I used UWorld exclusively for questions.
That being said, to use PreTest is usually a good idea. The problem I found is that they lag behind the popular trends a little bit. Some of the questions are nuance or factoid based, without much thought put in to the vignette. The explanations are usually better than the questions themselves, making it a useful study guide, but a poor practice material. If you have the PreTests and like them so far, keep going. If you are trying to decide between them and something else, I'd go with something else.