I've had a rotary sharpener for years and have never had a problem. I buy blades in a pack of five but don't need to buy very often. I love it.
I did buy a calculator but haven't worked it out although I do have a tutorial on how to use it, saved on my computer somewhere.
I have my machines serviced regularly. My last machine outlasted the company that made it. I had it over 30 years. Hope my Janome machines last as long.