I'm taking the view of an avant-garde designer in a firm that has a separate team of drafters to implement designs and construction documents. My focus is on the conceptual design phase, so I simply have different needs.
There are a number of programs, but most designers I know prefer Rhino (rhino3d.com) and the Grasshopper (grasshopper3d.com) plugin. Rhino is now adding Python (and OSX) support, but they still have a ways to go.
Never mind my other reply, I hadn't seen your response above. I had a quick look at Rhino, I hadn't heard about it before. I'm a bit surprised that you work with the same tools as e.g. an industrial designer, although when I think of it I can understand, with the lack of expressiveness in the specialized packages and all.