The good: Google SketchUp is free; elegant, with an intuitive interface and flexible 3D modeling tools, instant learning curve; tight integration with Google Earth.
The bad: Google SketchUp doesn't allow you to push or pull curved surfaces easily to make them three-dimensional; currently Windows only.
The bottom line: Google SketchUp is a flexible, powerful app for quick 3D sketching on the fly, but professionals will want the $500 version.