I felt like Qbook was excellent for my internal medicine shelf exam as an adjunct to MKSAP-2. I also used Qbook for my peds exam, which was less helpful but worth it if you have the time. IMO, it is a good review of high yield material but the questions are slightly easier than those found on NBME shelf exams. BTW, bulldogs kick a$$!!
Q-BooK has gotten positive reviews, in most of the threads/posts in which it has been evaluated. It has not gotten as many reviews (good or bad) as, say, USMLEworld, but the majority of comments that I can recall have been positive!
I liked it for medicine and surgery OK but I really thought the OBGYN section sucked. The stems would be reasonable but the question and answers would just be so dumb I can't imagine the real test is that easy (step 2) - I know the shelf was not...
I personally really enjoyed the Qbook. I did it before most shelf exams and I felt I saw about 2-3 very similar questions on the shelfs for each. It's a good way to test your handling of the very high-yield/basic information.
I also felt it was a pretty strong resource for Step2. By no means a replacement for UW, but it covers a lot of testable but forgettable topics - like fibromyalgia vs. somatoform d/o etc.