The Test is easy, you have to get 15/20 to pass.
Read through the discover canada book a couple of times and I used citizenshipcounts.ca for practice tests.
The interview consists of asking you for your original documents, a brief discuss about whether you are employed or in school etc, fairly standard stuff. However you wouldn't believe the amount of people who didn't bring documents that you are specifically asked to bring (ie passport, PR card etc.).
My interview literally lasted about 5 minutes, but I'm British so they probably had a fair idea that my command of English would be good.
The only thing I would recommend is plan on being there for 3 hours. The interviews are done in alphabetical order and I had to wait an hour and a half after the test to speack to the CIC official, so make sure you put plenty of money in the parking meter
Nothing to stress over though.