The CRP-5 is not difficult. If you are taught in class or by your instructor to use it (as opposed to self-taught) you should be shown the reminders that are on the computer itself. In fact taking the thing in to an exam should be regarded as cheating - it effectively tells you how to make each calculation, if you have been shown what to look at!
The CRP-5 is a very fine computer, better than my RN-issue Daltons or my Jeppeson (though that, being aluminium is very robust). It is also the one the JAA use to work out the answers, and since some are very close together by the CAA's admission, should be the one all ATPL students use.