First off let me say that I really like MoI. I've almost never used a spline modeling tool before, and in just a few hours I am happily modeling away:-)

My only problem at the moment is that I find it hard to predict if the filet tool will work at all. For example, I tried to create a simple shape with a button sitting flush with it. Here's my workflow:

Create a spline and extrude to the shape I want.
Create a curve and trim the shape of the button. (In order to keep surfaces for both button and shape.)
Select trim curve and extrude to use as sides hole in which the button goes.
Select extruded sides and join with trimed surface in order to create a filetable "corner".
Now when I select the "corner" curve and try to filet it, it will usually fail no matter what radius I set the filet to.

Is this a weakness of the filet tool, or is there something substantially wrong with my modeling workflow?