If it's OCPJP 6 and not 7, I think it's the best simulator for you.
Obviously prior to that you should read the famous K&B OCP 6 book which is a must in order to aim at a good preparetion for the exam.
Once grasped all the concepts explained in the book, go ahead with the above mentioned simulator. (it's also on of the few simulator for free and high quality)
I personally would recommend also this website http://nikojava.wordpress.com/2008/10/10/free-scjp-mock-exams/