Thanks All! Maybe my post is misleading that the exam is very hard... excuse my English, it is not my mother language. To clarify the difficult level about the exam to the folks preparing the exam, I'd say it is as hard as SCJP if it is not a little easier than SCJP. My SCJP is 86%, my SCWCD is 88%.
By the way, before taking the exam don't forget try Anand's mock and the one in http://enthuware.com/jwebplus/,
they are very similar to the real one.