I love fast fret and use it on everything
Fast fret is essential for me as I play constantly (pretty much every Thursday-Sat and very often Sun-Wed) and I have an abundance of very acidic sweat. Fast fret has made a world of difference for me in getting strings to last. I use it both on the fret board when I'm changing strings (just a quick once over to make sure there's no crap building up between frets) and then a quick once over after I'm done a gig.
ProsStrings last longer and play better.
ConsStrings feel pretty slick (which I like, but it's one of the first things people say to me when they use my guitar).
That damn fluffy shit gets everywhere so apply it somewhere outside of your home (especially when you get past the halfway mark on the fast fret applicator).