After having postponed taking the exam for almost 9 months, I took it yesterday scoring 93%.
I have began preparing for this exam in September last year. My first intention was to sit on November/December, but had to interrupt studying due to personal reasons. Then, when I again had the time, I forgot about SCJP 5.0 and went directly for SCWCD.
The exam consisted of 46 questions and was easier than I thought. The time given (1h40) was enough for answering and revisiting all the questions. I was expecting a lot more questions about Generics and API, but in fact there weren't so many. I had a lot of questions about language fondamentals and some about threads. This time I missed 3 questions (one of them on Collections and Generics, a point I thought I was very strong) but I am really satisfied with my score.
For preparing, I passed thru Philip Heller and Simon Roberts' book (Sybex). Being it a book intended for people preparing for the SCJP exam, I found it difficult to get to the topics I needed for the upgrade exam. I feel that my task would have been easier if I had used K&B book. I completed my preparation using Sun's tutorials on the new topics (specially for regex and generics). A very helpful resource was Angelica Lange's FAQ on Generics: it helped me understand what is happening behind the scenes when using generics. For studying and memorizing the API, I repeated what I did for the SCWCD exam: I prepared a UML class' diagram of the relevant objects with the relevant methods.
Finally, what I think really helped me a lot was doing a lot of mock exams: Marcus Green's free exam, JavaBlackBelt's, Java Inquisition, Whizlabs free demo and the most important of all: Enthuware's for CX310-055 which I found worth the price and really useful even if it is not directly intended for the upgrade exam.
I want to thank all the people in the SCJP forum for their help. Java Ranch was a crucial factor in my preparation (I've been reading the SCJP forum at least twice a day).
Thank you again. Now it is time for my holidays and then... I'll go for the SCDJWS certification.