Many thanks to Dick Steffens, who has very kindly taken the time to review Varkon as a potential CAD system for the RepRap Project. He sent a report which I've put up on the RepRap site. Get it from the Reports, results, and documentation link. Here is his summary of the report:

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In an e-mail, Adrian suggested that the two main criteria are:

1. Ease of use for precise dimensioned mechanical design, and
2. STL (or OPenRP) file output.

"With experience, Varkon probably can be precise. In the short amount of time I spent looking it over, I learned that there is a way to import and export .dxf files, but I didn't see anything about STL or OPenRP.

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I (Adrian) have been looking at Art of Illusion (recommended by Vik Olliver) and at Blender (recommended by Michael van der Linden). AoI is certainly the easier of the two to use, and seems pretty powerful. Neither is designed as a CAD system; they're for 3D graphics and animation. But both have STL output (slightly experimental in the case of AoI) and so RepRap can use them. Now I've got the hang of them I intend to use them in anger to design the same part (a screw feed for a polymer injector for the RepRap machine), and see how I get on.