The Planet Waves S.O.S. is an interesting tuner that offers users a different method to tune their guitar or other stringed instrument.  I have only used this for guitar and for playing at home, meaning I haven't brought it to any gigs as I use a Boss TU-2 tuner for live shows and that it also what I use most of the time at home.  I got the Planet Waves S.O.S. for free with a few packs of strings and was eager to see how well it would work, but was a bit surprised that I couldn't get the hang of it right away.  While it is a little bit tricky to understand at first, once you get the hang out it I think that you'll enjoy using it as it making tuning pretty easy.  There isn't much to it, but you have to make sure that you can read the light properly to gauge if you are in tune or not.  While it is a unique tuner, I do not think that it is the most effective method.  Whether you need a tuner for home use or for gigging, I would recommend going with something a bit more reliable like a pedal version of a tuner, as I find that that is the best method for myself.  The price of this I do not really know since I got it for free, but since I got it for free I can assume that it won't cost all that much if you want to pick one up and while it is a cheap option for a tuner, if you are using it for live shows I would strongly recommend getting pedal.  This is definitely worth a look at but to me, while unique, this isn't the best method for tuning your guitar.