I've been without an electronic tuner for my acoustic until now. This thing is fantastic. I can't really think of a way it could be better; it reacts instantly to tuning and is small and light enough to leave attached to my guitar semi-permanently.
I don't think a guitar shop in the world exists without a bunch of these for sale for around $20. They do such a good job. They're easy and simple to use. I think every instrumentalist should own a Snark.