I have to agree with Genghis about the Aristo Aviat slide rules. They are they very best that money can buy. The CRP-5 is a nifty piece of work as well but when you see an Aristo Aviat you'll see where Pooley got the idea from. The E6B is poor relative of the CRP-5. Nowhere near the quality and it has a poor layout. The CR-n computers do the job, fit in your pocket but never get used as mental maths is accurate enough in real life.
As for their use in exams, I found that I didn't really need a whizz-wheel for the CPL because your rough 'order of magnitude' and 'units' workings before the number crunching was close enough. But that was 20 years ago. I also found that most but not all ATPL questions could also be answered with just rough workings. As to what you should buy now to churn out the guff required the Euro ATPLs, a person like Alex Whittingham would be the best to give advice.