I've been using Carrara, well since it was Ray Dream... REally LOVE to Spline modeler for doing more Mechanical detail items as I can draw sections very precisely inside programs like illustrator  and such and import to get really nice models.

One kinda major issue I have found is that circles and toruses are very jaggy no matter what i do.   Ive gone and actually increased the point count and such in Ill-y when importing and when doing easy stuff in Carrara and just using a simple torus it comes out jaggy...  something I'm missing?  the image included is just a simple profile with torus applied in the spline modeler (PS  It's Carrara 7 Pro as I simply can't $$$$ to upgrade and honestly I don't see anything important to me improved in 8 at this point)

 

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Hexagon, Carrara, Sculptris, and recently Sketchup.