yep for cheap, cold pour soaps it works well. i've mixed 1 teaspoon of glyerin to two pucks of surrey/van der hagen soap from walgreens and it improved it greatly.
I've added a small amount (.25 teaspon) to puck of Williams Mug Soap. Not melted into it, just poured on before hitting it with brush. Yields a decent, stable lather....as compared to regular lather with williams soap, which is weak and quick-drying.