Always use mine too, the CRP5 & 1, but never got near the 300kts compressibility point in a PA-28, something to do with VNE.
Never had to change the batteries in 7 years of using the CRP1, and it has never had a programming glitch giving eroneous output.
There are some things you can do with a CRP quicker than a calc, such as fuel weight.
It also doubles as an efficient fan on a hot day, Casio are still to market the Calcufan.
