Getting geometric shapes that smooth at the edges and smoothly fillet 
together at joints, such as you find with automobile bodies, is a matter 
of constant invention and modelling skill.  Basically it is done best 
with NURBS modellers, like Rhino, (since I think that long before 
computer graphics, modelling prototypes for automobile designs was what 
splines were originally invented for), or the more recent polygonal mesh 
modellers, like Wings.  In POV this can be achieved also through 
dexterous use of the bicubic patch. Many do this with hand coding.  A 
modeller that produces results in  bicubic patches is Hamapatch, 
successor to sPatch.

If you want to use solids your options include some use of differencing, 
and intersection to try and approximate the smoothing you want.  There 
are various macros for producing rounded boxes from spheres, cylinders, 
and boxes for instance.  There is also a rounded box available with 
isosurfaces.

Isosurfaces give the means to get a variety of geometric shapes to 
smooth together like blobs.  The blob primitive itself only supports 
cylinders and spheres.  With inventive use of scaling the basic blob can 
go a long way towards a believeable autobody.  The use of isosurfaces 
could push it farther.

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