Groskamp does (did) use them as well.

He discusses in detail how much he sands them in his videos. There is also a video on the petitrc site with sanding tips too (edit:link now below) - they sand them much more lightly than Jilles does. Making the plastic ones the same size as the metal ones - if you have them handy to measure - is a good idea though.

Does anyone have the metal ones to hand (without pulling apart their diff!) and can measure them accurately with a set of
verniers?

http://youtu.be/NXZM4mdlUAw