Although I haven't used their SCDJWS simulator, I had used their practice tests to prepare for my SCWCD and SCBCD exams. For the SCWCD / SCBCD exams, I found their simulators to be very helpful and a good reflection of the types of questions you'll find on the actual exams. In fact, I passed the SCWCD / SCBCD on my first attempts, and scored 92% and 88% respectively.
When I take the SCDJWS exam in the next 6 months, I'll defintely give their SCDJWS product a try. Hopefully, they will come out with their SCDJWS simulator by then.