I have only recently started playing the guitar, but have been playing other intruments for a good few years. I am pretty hopeless in trying to tune my guitar by ear, so this little tool is perfect. Plus its a two-in-one if you have a bass guitar as well.
I am not sure what the 1-star review for this product is about, I can only think that their unit was faulty or something. After tuning my guitar, all the strings ring beautifully and I repeat it every few days. A good investment for guitar player experienced or beginner.
I would recommend you reading your the instruction manual if you are new the the guitar or not sure about notes/tuning in general. Its helpful if you know which string should be what note before tuning.