I am new to this board so I want to share my Clep experiences to date. I used the instant cert materials to study for the Humanities Clep exclusivly and passed with a score of 67.
American History II - passed 56
Western Civilization II- passed 56

Both of these tests were not hard. I am from Europe and never studied American History prior to Instant Cert so I think that resource is quite good. I got a score of 56 because I only studied for two days prior to the exam.

I am taking American Government and American History I next week and I am using the Instant Cert website along with the Peterson's practice tests along with the official Clep Exam Guide. I will let you know how I do. These tests by the way have a lot of questions that are very tricky.

Have a great day!