As to what it takes to get through it (CPS or CPI) is knowledge and experience with the WinRunner product.
I does help to run through the tutorial, and to take the Mercury class.
I'm working (off & on) on the CPS myself and it isn't easy, but I do believe that it will be worth it once I've gotten my certification.
Please remember that Mercury requests that you don't seek direct help with the exam from outside sources. They do have a help link on their site that you can contact if you have problems with the exam.