The CRP-5 is an excellent tool. If you know it like the back of your hand, it will save you time.
Formulas are all very well, if you have the time to remember them, and yes, they are probably more accurate, however that level of accuracy isn't really required.
Also, I find using the CRP-5 with one hand easier than using a calculator with one hand.
If you find formulas quicker, then use them, personally, I prefer the CRP-5.