I just passed my first java certificate with 83%
As for preparation it was actually quite fast, because I needed certificate to start new project for my company.
First of all I have about 3 years of experience in java, but during preparations to this exam I realized that, without Eclipse it was hard for me to read code and point errors. I lacked real basics of grammar.
I read 'OCA Java SE 7 Associate Study Guide' once.
Then I took Enthuware exams, one per day, and after each exam I studied my answers and noted explanations for questions answered incorrectly.
Each next day I read notes from previous days, and then took another mock exam:
Starter test: 65%
Final Test: 79%
After final test I used Enthurware in Leitner Mode for couple of days.
I was worried about my results on mock exams, but I had to pass this exam before September... and today I did it.
In my opinion real exam was easier than Enthuware exams... but I barely managed to finish on time. I was able to finish each mock exam about 10 minutes before time, carefully studying each question, but today, when I reached half time I was on 24th question. Almost all questions required to anticipate output of given code (mostly over 20 lines), unlinke in Enhtuware, there were very few 'true/false' or strictly theoretical questions.