MOI is working out fine. It's just little abstract in extreme situations. But I'm really getting it, most of the time :-)
For example to create the object to cut out the front hole where the iphone sits in the iphone dock: http://pocketables.typepad.com/photos/uncategorized/2007/06/30/iphone_dock.jpg
I hav tried a thousand ways: cutting an box with straight extrudes from 3 directions, making a mesh with poly's, etc... nothing realy works out fine.