A Tack-o-Meter is just a fun add-on. It won't make you go faster, and might even slow you down with that extra weight. It's also just something else to clean and wax. Plus you've got to consider the extra drain on the electrical system to power the light bulb...might be risky when you plug in your cold weather riding suit.

Aside from that, for the little extra expense you have something to verify idle speed and to know whether you are actually in 5th gear or not.....3500 RPMs at 50 MPH.
Mine gives me a secure feeling and peace of mind