Hey John,
I don't doubt that C3D is a great bit of software, but my only and rather major gripe with C3D, is that C3D is unit-less, i.e. specify a line as 1m in SU, and you get something very different, and rather illogical in C3D making it almost useless for architecture- unless Cheetah's sole use is that of a rendering application for use with SketchUp. Even then, once you have exported a model into C3D, you are faced with billions of faces that all need grouping, and that's before UV mapping and the like.
I suppose it all boils down to what you intend to use it for. As a straight forward modeller, C3D is great, but personally I'd rather save, and later invest in something a lot more powerful like Cinema 4D. I too have the Rhino beta, and SharkFX, and although I do like Shark (because it reminds me of Solidworks), working with Rhino is like running through toffee! Perhaps it is early days still with Rhino? The PC crowd swear by it! The SharkFX community, although small, are some of the friendliest folk I've ever met online (in the CAD community).
I hope that helps