Hey thanks Coderanch, I got re-certified today (last did it in 2001). Really helpful resources on this site!
I scored 86 (did the full exam, not the upgrade) which was about where I wanted to be, i.e. not to overstudy the details but be sure a more general grasp would get me through.
The materials I used were K&B (OK) and K&R (really good). Printed out the Frits notes of K&B(on materials OCJP page) which are absolutely critical. Plus I bought the Oracle eCertify practice exams and used free other mock exams linked from the coderanch page.
I have to say regarding the K&B book: it is GOOD for understanding what the exam covers, but it's too scattered and rambling to be a good study guide by itself. I think if you're relatively new to Java it is a great light read but don't pound your head against the wall trying to get understand nuances. The details of many topics are just not presented in technical enough language as to be precise enough to leave no doubt how all permutations of code will execute. But it really shines if you need a readable, less dry intro to one a Java topics in the exam objectives you're unfamiliar with. It provides good questions for practice, but they tend to be overly tricky and can distract you from the core lesson being taught. Don't be surprised if you find some of the K&B questions difficult to answer by referring back to the chapter: the details are sometimes too buried, elsewhere (another chapter) or not really there at all (i.e. it will introduce a detail only as an explanation for why a particular answer choice is correct or incorrect.
K&R on the other hand is too detailed for the exam, there is way more information than you need to pass the exam. However it is much much more concise and better laid out than K&B especially when you flip it open to get to exactly how a particular aspect of Java works The other GREAT thing about K&R is the index is very thorough and before treating a topic it will reference you to where to find the pre-requisite material (in the book: section & page #) so you can know for sure you're ready to tackle a given topic. K&R also does a nice job of isolating a sub-chapter's lessons into a set of practice questions aimed directly at the material in a small # of pages, and the explanations of the answers are also very good.
RE The Oracle eCertify practice exams: I figured they'd be the closest to the real thing but there are better values out there. The questions on eCertify are IMO slightly more difficult than the real thing plus there are duplicate questions and flat out errors in the scoring, but probably worthwhile to gauge if you're actually close to ready to take the exam.
If I had to do it over again, I'd very quickly read K&B... doing end of chapter questions but not worrying at all about missing the gotchas, reading still carefully explanations for the answers. Then study the Frits notes of the K&B book. His notes are a really good condensation of the scope of the exam (though missing a few minor things here and there which are covered). Then take some practice exams (there are plenty of them out there) and see where you're missing the picture on how something actually works (e.g. constructor chaining or method shadowing). Also, the mock exams will give you a good idea what needs to be memorized vs what info you'll likely get from a question's content (or from another one on the exam) and therefore which details of the API you'll be expected to know. If you need overview, back to K&B ... otherwise for the details and the more technical mechanics go to K&R and/or the online Oracle Java tutorials.
The exam questions aren't nearly as tricky as you're lead to believe by reading K&B. But the exam does throw a couple of gotchas in there: use a var out of scope, that sort of thing. But generally not in combination with really difficult topics like generics or threads or the nitty gritty of a facet of Java which is already a subtlety. It tended to be more like: you see a straightforward looking piece of code... and it asks what is the result and that's when you should look hard for the gotchas like an access modifier being wrong or the like.
Anyhow good look to all who are preparing, I know there are lots of these de-briefings out but maybe somebody will nonetheless stumble on this and find it useful.
FYI I scored a 65%ish on the K&B mock exam, and 69% on the K&R, and about 80% on the eCertify.