I use a glo-bee and haven't had any problems with it as long as I'm outside - inside they pick up the florescent lights flickering and give false readings (then again, you shouldn't run a glow engine inside anyway).

Having said that I'd never use it for tuning a single engine since the temptation is too great to tune for max rpm and have the engine run lean in flight. What I do use it for is synchronising twin engined planes (though the ear's probably more accurate than the tach for this), and comparing performance with different props and fuel mixes.