If you haven't looked over the Cattle Drive site, go
here to check out what all is involved, and what topics are covered in the assignments.
Cattle Drive is about writing code that is readable and efficient, so it isn't geared directly for the
SCJP exam. However, after doing the Drive for several months, I passed the exam in Jan. with flying colors, so I would say it definitely prepares you for it better than just reading and trying to write code on your own. Matt Phillips is also a Driver, and he passed the exam. Yes, you still have to do a lot of experimenting and learning on your own, but at least this way you get someone looking over your stuff personally and telling you what works, what doesn't, how to improve, etc.
So after all that, I would say yes, the Drive helps a lot in learning to program with
Java, which is what the SCJP exam is all about.