I've always considered Sheldon's tool a bit of a kludge and to be used only when the entire bb, cups and all, is to be replaced. If you want to reuse the cup you have to be careful that the nut that goes inside is small enough that the corners of the hex don't dig into the actual bearing track. And if it is small enough to avoid the bearing track it may also be too small to grip firmly enough to break loose a really recalcitrant cup.