Bought this to get my G Les Paul & ESP fretboards back to their previous lustre, it does the job very well. As with any product like this you'll probably have to reapply after a few of gigs to counteract sweat dry out (from your playing), but that's part and parcel of guitar maintenance. Best used when you replace the strings.
If you have a valuable and / or well loved instrument with a fretboard needs proper care you will need some of this. It's that simple - and yes it does work. You can try alternatives like 3 in one if you are desperate but this one smells nicer and works better.
I have this and I have some Dr Stringfellows Lem-Oil. I prefer this one as you can decant and rub it in - better than spraying it everywhere like the Dr Stringfellows stuff. I'll stick to this product going forward. You still need to find something to lube the strings after cleaning thre fretboard otherwise they get dry and dull. Haven't found a suitable Planet waves product for that so will probably stick to FastFret until something comes along. Watch out for the string cleaner - not very well suited to guitar cleaning application.
There is only one problem I have with this thing. there isn't enough of it apart from that its ok, when you first put it on the wood shines beautifully and then it fades back to the way it looked before.
I bought this for my guitar to give it a good clean up. It should be used on the fretboard only, applying it to the guitar body or anywhere else that is lacquered will just leave it greasy. I have a rosewood fretboard and using this and a microfibre cloth, left it with a beautiful clean lustre.
Some lemon oils come with warning saying that they shouldn't be used on maple fretboards, this one in particular does not have a warning on it, but I would still do some research before thinking of using it a maple fingerboard.
The easiest and most convenient way to apply the oil is to do it when you are restringing the guitar. when I did it, I removed two strings at a time and put a few drops onto a microfibre cloth, I then worked my way down from the top fret, put some more oil on the cloth and worked my way up from the bottom. After that, I used the cloth on each fret individually (no need to apply more oil as there will be some remaining on the fretboard) and cleaned with side to side motions.
You know it works because you can see the dirt on the cloth afterwards, and you'd be surprised by just how much there is!
Not cheap but this will restore almost any non lacquered fingerboard to its former glory. It closes the grain & if used at every string change it will prevent cracks in the rosewood etc from happening in the first place. Good stuff & lasts longer than you'd think.