Grout sealer enhancers do definitely work. The use of sealer enhancers on grout not only brings out the color of the grout but it also helps the grout avoid discoloration. Grout sealer enhancers may not work as well on certain grout colors, but will enhance usually any color it is used on.
No, I've tried several different brands on my tiled kitchen counter and none of them have held up to the test of time. Perhaps, it is the moist environment; but a tiled bathroom would also be moist so I am not sure that is the reason.