I think to use only one study source is a bit foolish, though I think in many case, instantcert is plenty enough to pass. I like instant cert because it helps give me a good outline of study. I can then read a secondary source and have a much better idea what to look for and where to focus a bit more. You may want to give the administrator's some input that can help improve some of their study material. I think setting up this discussion board if a very beneficial move on the instantcert administrator's part.

I always use an additional source. I have no room to fail an exam. Nor time or money to fail.