I have carried a Jep CR3 for nearly 20 years now, and I still use it for fuel calculations and such. Once a year or so I complete PNRs and Mach calculations just for fun, but I don't need them here in Europe on the 737. In Oz on the steam driven aircraft the calculations were useful.
When I needed to complete the 14 JAR ATPL exams I was advised to buy the CRP, as "the Jep wouldn't give the correct answers." A little research showed that the Jep is set up for a recovery coefficient of .8 (matching most temperature probes on jets, certainly on Boeings) while the CRP is set up with a recovery coefficient of 1. In order to get the jep to match the CRP it is simply necessary to add an extra step of dividing by .8
I used the jep in all of the JAR exams, and passed all of them.