I have not taken CK yet so cannot comment on how helpful they are for CK prep, you are better off asking that q in the Official CK exam experience thread. But I have tried to do 1 form before every shelf exam set by my school, and they are a fairly good predictor of where you stand in terms of knowledge-base. I have to admit some of the questions are definitely challenging, and great practice as they are actual NBME questions. I just wish they gave an answer key at the very least 🙁