I have the Globee. I have found it to be accurate when used in a particular manner. That is, hold it close and at a 90 degree angle to the prop. Close is less than 2 inches. I have found the Globee will pick up on reflections (flashes of light) from other shiny surfaces such as bright aluminum engine parts, and shiny cowl finishes. This confuses the photo sensor and alters the reading. Getting close to the prop blades eliminates this interference. Get the prop as much as possible directly between the tach and the light source. Inspect the alignment of the optical sensor with the hole in the end of the tach. It should be centered, and parallel with the plane of the surface of the hole. If needed, separate the case halves and align the sensor. Keep the battery fresh. Just because the display lights up does not mean the electronics have enough power to work accurately.

These types of tachometers are cheap because they use cheap photo sensors. They need close to ideal conditions to work properly.

If you want a very accurate Tach for less than ideal conditions, look at the TNC Tachometer. At least go to this site and read the technical write-up about Tachometers.
http://www.ulster.net/~tnc/tncweb.html

I have the TNC PT10S. It is about $100. But it is very accurate, stable, and will read from over 12 inches at dusk on an overcast day. It will register in conditions my Globee will not.