Is it terribly difficult to pass? What is the pass rate?
Depends on the person, I don't think they release out the statistics, nor do people often talk about how they went.
Is there anything I can do to prepare? How do you do well?
Driving in general is a pretty good way, that URL that the user 'drunk' provided can be used, but in general, it's more about clicking when it's safe to move or overtake etc. Also if you have glasses etc. make sure you wear them if you need them to see things properly, because even if you had no issues with your eyes, you can still have trouble seeing the images clearly in the screen (relatively commonly complained about). I have provided some information/tips below as well that will hopefully be of some use.
If you fail, how long till i can go for it again?
Assuming you fail, which I hope you don't, I think you can just rebook the next time slot available.
Tips:
You don't need to always click, sometimes, the answer is such that you shouldn't click if you feel there is no need to.
Don't be 'trigger happy' - i.e. click too quickly or click too often.
You will get some practice questions, go through them carefully, although they don't count there is a chance they will pop up in the actual graded one.
If there is a need to slow down, say from 60 to 50 because there is a sign, slow down
after before you've passed the sign.
http://forums.whirlpool.net.au/archive/809633 is a good thread to refer to, there are many good tips provided there.