Your experience may vary, I do not do well generally on standard tests without putting in a lot of time. And I have many years of hands on network troubleshooting experience. But I see others who sit down and pass a test on the same day we finish a training course. They just retain information in this format easier.
As you move up the specialist and professional exams are much harder to pass if all you have is the study guide. In my experience the study guides are great outlines of what is necessary. but I need to reinforce the knowledge with more detail and preferably with configuration practice in a lab.
You can see where you stand at the start of your studies by taking the practice test on the education site. As you go throught this they give you the correct answer so you can see what areas you need to study. This site also has the master outline of topics for the exam.
http://www.juniper.net/us/en/training/certification/e_track.html#jncisent