I used it, quite a while back, but it was really pretty good. Our library had a subscription to it so it was free for us so I'm not sure cost-wise. Explanations maybe were a bit briefer but I don't recall exactly. In general though, do as many questions as you can get your hands on.
I used BoardVitals, Kaplan, and UWorld for CK (as well as Step 3). Here's the thing - make sure you leave enough time for UWorld. IMO it's still the best resource.
That said, BoardVitals is on par or better than Kaplan. There were a few questions that were on the exam almost directly and the explanations are good. I consider it to be the #2 question bank right now and it was a pretty big help. (I'm using them for my in-service exam also)
The qbanks are better prep than a book, especially since I could not stay awake. My residency program weighted CK pretty heavily in the process, so it's worth spending the time on it. Good luck!