Well, the battery in this one can be replaced, although it is covered in some kind of gooey silicone gel. We'll see how big of a PITA it's going to be to actually replace it.
The RPM display isn't very useful on a mower since most mowers are designed to run at constant RPM these days with no throttle control. My Toro (B&S 190cc engine) shows about 2800 RPM. I guess it could come in handy if one needed to adjust the RPM, but again, I'm not even sure if that's possible.
When I hooked it up to my motorcycle, the RPM at idle was fluctuating quite a bit and there was a lot of delay in the display when I cranked up the throttle. So, for real-time RPM monitoring, this $20 meter is no good. Alas, I mainly bought it for the timer functionality.