I felt that UWorld was not the panacea for all shelf exams.
For instance:
If you are taking the medicine shelf, do MKSAP questions (not similar to the exam but a good experience)
If you are taking the OBGYN shelf, do the ACOG questions
Case-in-point: I used UWorld for the pediatrics shelf and my score was secondary to reading and NOT UWorld questions for the most part.
I think the best preparation for Step 2CK is studying hard during your MS3 rotations. UWorld gives you good prep for Step 2CK, but there are going to be questions on Step 2CK about certain subjects like peds, OBGYN, etc that are not covered in any qbank... they would have been covered if you studied.
I don't have experience with using the Kaplan Qbank for Step 2CK. I did use their question book, which was good in some parts during the MS3 year as supplemental questions. However, I think UWorld has the monopoly on the best qbank for Step 2CK for now... probably because the questions they create are similar to the NBME practice exams (and are subsequently phrased 5 different ways throughout the qbank for repetition).