I took the Sun Certified Java Programmer 1.4 (SCJP) exam this morning and passed at 95%.
The actual test questions are easier than what I had expected. The difficulty of multiple selection question is dramatically reduced by giving away the number of answers expected.
The main book I used for self study is A Programmer's Guide to Java Certification, which has been my trusted Java reference book for the past three years. Since the book is written for SCJP 1.2, only the first eleven chapters are relevant. For test topics not covered in the book, I used Sun's assertion tutorial and the chapter 3 of Effective Java for its coverage on hashcode() and equals().
The mock up exam on http://www.danchisholm.net covers the test topics throughly and provides detailed answer. Much recommended. If you are an experienced Java developer short on time, you can probably skip all the book reading and start right from there.