Not a spray! Contains no silicone. It’s a liquid in an applicator. Use it on your strings, back of the neck and the fretboard. It lets your fingers slide freely, keeps your strings clean and conditions the fretboard. (Quote from GHS website, July 2016)
I use Fast fret on all of my guitars straight after using them. I think it is a really brilliant bit of kit, that in my experience makes strings last longer and play better and more easily. You use it by rubbing the fast fret applicator (kind of a stick with a block of material that has fast fret solution soaked into it) along your strings and then wiping off any excess with a cloth.
When you buy a pack of fast fret, you get the applicator and a cloth in a little metal tin. I’ve generally found that the cloth leaves little bits of fluff on the strings, so I use a special lint free cloth that I bought in Wilkinsons for 99p which works brilliantly!
Fast fret works really well at keeping strings clean and playing well, when used regularly. However applying this after you play is not a substitute for regularly changing strings and conditioning the fretboard. Whenever I change the strings on my guitars I like to give them a wipe down with a slightly damp cloth, quickly dry it and then I check for any bits that need tightening up or any repairs that need to be done and finally put some lemon oil on the fretboard (if it’s rosewood, don’t do this with a maple fretboard!) before finally putting the new strings on.
The fast fret solution, is not the only product on the market that cleans strings and conditions the fret board, there are many many options available and all of them do things a little bit differently, but much the same really. I’d say it’s really important to use something to clean your strings and condition your fretboard, but you can choose what you want to do it with. I however, will be sticking with fast fret.