Inventor is a completely different animal then AutoCAD. The modeling is a completely different way of thinknig than AutoCAD. Everything in Inventor is dimension based as opposed to object based (to stretch an object, you change the dimension, not offset faces.)
My company recently bought the "Inventor Package" Which includes Inventor and Mechanical Desktop (as well as plain old full 2005) And right now I'm using 05 to get by and trying to work in Inventor where I can. Your best bet is to get the free Trial Version from autodesk and then go to a site like avatech and participate in a webcast. They do a great job of demonstrating the functions of Inventor. Hope this helps.