I didn't like the workflow of left click to lower and then having to click another button to raise. To my mind this is a little bit retarded. You can probably use a wacom pen as in code archives, where the eraser is actually pull and pen is push. Or vice versa. Pressure sensitive...
We did the two button approach in UnrealEd for terrain editing and it works wonderfully. Left button increases the current tool, right button decreases it. This applies in a generic way to editing the heightmap, painting textures, smoothing, ets.
Having to constantly switch modes to raise/lower is a productivity killer.