If you are asking me, as I said though I initially registered, I cancelled it later. I hadnt attended a single class yet.
It really depends on what you are looking from the prep course. If you are looking for good practise material, there are tons out thr. Infact just relying on kaplan or another course may be risky !
Prep course for one can help if your basics are very poor, you would be able to discuss and learn from an instructor. Another reason to join may be because you are not motivated to study regularly without a classroom setting.
Do visit Eric's blog:
https://beatthegmat.blocked , It is a wonderful resource for planning strategies for GMAT preparation.
Hope that helps !