I've only authored one part, 92579, but I did use SketchUp for most of it (here's an example screenshot). I don't think it's a common tool among part authors because it doesn't inherently know about subparts or primitive parts, but I found it easier to use than writing LDraw code for a new part from scratch. (No doubt the LDraw masters here can "see the Matrix directly".)
Anyway, more info on my Sketchup to LDraw conversion process here.
Anyway, more info on my Sketchup to LDraw conversion process here.