My son and I have used one to hatch one russian tortoise
and several each of leopard geckos, day geckos, and knight anoles.
1) Get a digital thermometer with a probe. The little glass thermometer that comes with it isn't easy to view. And you can reduce the number of times you open the incubator by having an external read out.
2) I think the instructions tell you to pour water into the bottom of the incubator. I think it's better to add a small dish of water. Otherwise the bottom can get messy.
3) Set it up a few days before you need it. It takes awhile to get the temp just right (the method of adjustment is kind of crude) and then it takes awhile to stabilize. Let the incubator go for a couple hours before making adjustments.
4) Set the incubator up in an area with stable temperatures. Not buy the door or window, or next to a vent, etc.
The above applies to the bottom of the line hovabator I got. It had a small plastic window in the top, a cheap thermometer, and no fans. I've heard that the model with fans will dry out the eggs.