Can't decide whether to use my boost on my main alt, a level 80 hunter or level him the hard way and boost my shadow priest that is sulking down at level 30. Seems a waste to use it on an 80 but levelling is my least favourite part of the game nowadays, it's more like a chore you have to do to get to the fun stuff!
The amount of time it takes to level you may as-well do the 30. Although if you did the 80 it could do professions also. Personally I think it would be a waste.