I just took the SCJP 6 exam and wanted to give my impressions of the experience.
The first thing I noticed was that the monitor refresh rate was slow enough to be quite noticeable - some people may find it very annoying and/or fatiguing on the eyes. The chair is also very uncomfortable. Just FYI - I've been spoiled by comfortable chairs and monitors made within the last decade, so it took me a little extra determination to spend the time I needed on the questions.
The test suite also froze about halfway through when I was working on a drag-n-drop question. No worries, the answers were saved and the proctor simply had me use a different test station.
The drag-n-drop questions also cannot be reviewed in the sense that you can check your answer - basically when you review it, you have to re-do the question. (It prompts you with a "this question already has an answer, reviewing it will require you to answer it again" message).
On many questions, you click a button to pop up a sub-window containing code. This second window has a "tile" button, which reshapes the windows so both take up about half the screen. This was sufficient in the large majority of cases to have everything displayed on the screen.
What's nice is that each question will tell you how many correct answers to choose - something that slightly annoyed me about many of the free online mock exams (A poorly-worded question could be ambiguous, leading to different answers - not good!). There were many questions where this fact helped me verify my answers, and then I could feel confident in the answer and not feel the need to mark it for review later.
My test seemed to have a lot of questions on generics and threads. I was expecting the drag-n-drop stuff to be a pain, but I rather enjoyed them and I seemed to get quite a lot of them too. It seems like there were only a few "tricky" questions, i.e. a deliberately omitted pair of braces for an if-block - I thought there would be a lot more.
I got one question that was specifically about the format/printf functionality, a few on serialization, and one about initializer blocks/constructor call ordering. It seemed like a slight majority of questions would only test on one specific topic. If a question tested two things, it seemed to test the more fundamental topics, e.g. assignments and flow control, or maybe loops and some operators. It seems like they wouldn't have a question that tested, for instance, both serialization and overloading, or both file I/O and generics.
I also got at least 2 (I can't quite remember now) questions on IS-A and HAS-A relationships, using the drag-n-drop format.
There were two questions specifically on static imports.
There were one or two questions about file/directory I/O things, and two or three questions about using the java command line program with classpaths.
One question (or maybe two?) on assertions. Basic stuff only, nothing about combining -ea and -da for different combinations of classes/packages.
Overall, it seems like the majority of the questions focused on the fundamental parts. There were no questions on bitwise operations or invoking javac with different source flags. I also got a lot less questions than I expected on collections - maybe only 4 or 5. (Keep in mind, I think the questions you get are selected from a random pool).