Dick has a good point and now after some experience with both glow and gas engines, I can hear when things are a bit out of whack, but I like to confirm things when setting up and then go by ear while flying to make sure things are okay. But if you are new, or can't really hear the differences for one reason or another, then a tach will definitely help.

BUT rajul is right... [8D]

When I first tried the tach thing I got one of those GloBees (sp?) and although it seemed to work pretty well, I did have problems with it depending on ambient light. So I got one of the TNC tachs and after comparing the use of the two in different light situations, different color props and differences in tach readings, I gave the GloBee away...