can't say anything about jdiscuss, did'n use them. I used the mock tests from ejbcertificate during my prep, and they were surely helpful in my opinion. They are very good in pointing out the parts you need to study again, and the ones you don't.
The mock exam that helped me the best, was the final one from Head First
EJB. I did that one the day before the real thing, and it's a really good prep !!
good luck !