I discovered this string lubricant many years ago and I concur with what most of the reviewers here have said about it although I wouldn't call it a string cleaner although you indirectly clean the strings if you use the cloth and like another reviewer stated you should also use the cloth to clean the undersides of your strings as well.  I unfortunately live in a very humid climate being only a little above the equator and so strings have a very short life here and so anything that can prolong their life is a real boon for a musician here.  I agree that the only downside is the inability to tell in advance that you are running low on lubricant until it actually turns dry but I guess given the price and the overall usefulness of this accessory, one cannot complain too much.  Now that mine has just run dry I'm going to delay a replacement purchase while I try out something new that I've just discovered called "The Swipe" which enables me to clean both the undersides and the topsides of my strings without chemicals.  If this works as well as advertised, I may still get Fast-Fret but for its lubricating properties which really helps make playing a lot more comfortable.