I wouldn't recomend them for making guitars. They can't be used to joint edges like a jointer or hand plane and if you try planing anything wider than the blade then it leaves these little 1mm steps.
The only thing i did use it for with success was rough carving some of the top but there is better tools for that.
They are also very vicious on the end grain.
A builder tool really, better used for taking large amounts of wood off softwood floor joists (on houses).
I would definatly go for a benchtop jointer instead.