They do work quite well. These are coating solutions- no magic involved. They are merely the clear component- the decal carrier film, that can be applied over old decals. The only downside is that they convert your decal sheet into one single decal. Subsequently, you must cut around each individual decal to apply it. Another minor inconvenience is that the resulting decal is a bit thicker than the original, so it may not nestle down around curves as well. The result is that you may have to use decal solvents then, and some of the decal film solutions do not play as well with some decal solvents as other brands.
BTW, be careful if you use any of these solutions- decal setting solutions and decal solvent solutions are NOT the same thing.