""""""""""The only drawback I notice when playing with MOi3D is the viewport speed.
When I start filleting objects the frame rate drops badly."""""""""

Rememebr that MoI is a NURBS modeler and you are making comparison to Polygon modeler.. They are different, especially when you start adding Finite detail like little fillets on everything...

In a NURBS Cad modeler comparison, MoI's Viewport blew doors on 15-20,000 dollar software!!!!

It would not be right to call it's viewport slow (Although I do undertsand what you are describing)

Usually, those other high end app's, start to "Cull" the viewport when trying to do any manipulation in it.."culling can be good, if you are starting to work on 1,500 part assembly's, but in everyday workflow, it is a distraction...

FYI.