Has anyone taken the certificate exam?
https://www.opennetworking.org/certification
Any exam prep materials, study sample questions?
No, and frankly I'd avoid it right now unless you have a working requirement. This "cert", in my opinion, is only one biased view. OpenFlow isn't huge in the real world - mostly academia right now. I don't see it even close to mainstream yet.
"SDN" is still very amorphous, and means much more than just OpenFlow in my opinion.
yes, definitely not meanstream, but armed with the knowledge from the SDN book i mentioned AND looking into the future:
http://www.networking-forums.com/programming-goodies-and-software-defined-networking/sdn-where-to-start/15/
I think it is worthwhile studying if looking into the future.
The book does not simply bias for SDN, it actually gives very detailed comparison of pros and cons of legacy vs. SDN and leaves user to judge himself.