I've bought it for the ebony fretboards of two of my basses. Though it is supposed to "work" better for fretted instruments, I had an extremely positive experience with a fretless bass.
Specifically, I mostly wanted to remove accumulated dirt and "soften" the markings from the use of roundwound strings on a fretless. There was some extensive rubbing involved, but the results were quite impressive. Don't expect wonders without buffing the fretboard, but... it was still very effective, and the fretboard was evidently rejuvenated.
I did use too much on my first try, but this was no problem at all; it does not leave any hard to remove residuals, nor changes how the fretboard feels.
All things considered, this has been a very good buy - I am not really experienced with such products, but this one seems to really do the job.