I read some reviews on these a while back. I seem to remember they were notoriously inaccurate.

When putting cigars in an airtight container like a coolidor you definately want to open the lid every so often. Cigars give off ammonia as they age and you need to exchange the air occasionally. Opening the lid every few weeks to check the humidity level isn't going to hurt the cigars at all. Besides, you ain't gonna be able to resist opening it up and looking in every once in a while.