I got one question wrong in EJB Overview, Session Bean Component Contract, Component Contract for CMP, CMP Entity Bean Lifecycle, EJB-QL, Transactions, Exceptions and Enterprise Bean Environment, respectively.
The primary basis for my preparation was the SCBCD Study Guide by Mikalai Zaikin. I don't remember any question on the exam that was not covered there. Don't forget to make a donation when you use it as much as I did! It seems to be the perfect way to prepare when you already know the most fundamental basics.
After each of the 14 sections I did a custom test with EJBPlus by Enthuware (a test with questions from just this section). EJBPlus is not perfect, but it is not very expensive, and alternatives are rare. I think that it does not cover everything that was asked on the exam (not unlikely that I am wrong, though), but by far most of it. I got 70 %, 71 %, 70 % and 85 % in the 4 complete mock exams that are included.
Prior experience was just based on reading the first part of "Mastering EJB", 3rd edition, plus half of the transaction-chapter.