I'm going to have to post back the results of my testing.

I've just played w/ DXF imports.  While they look good in Hexagon, they are 2D, not 3D as I expected.

3DS imports in 3D.

I've tried STL (Acad2004 export) but AutoCAD STL does not recognise a TIN, only 'solids'.  DANG.

I'll post the result of DWG14 when I get a chance to run it.