is the transition from autocad 3d modelling to inventor easy, what are the differences.would I pick it up reasonably quick

I have heard from some users of Inventor that it is nothing like AutoCAD and to basically forget what you know about AC.  It wouldn't hurt to attend a seminar of some sort or check out some of the tutorials over at AutoDesk.

I say yeah. It makes alot of previously tedious crap much easier, once you get used to the way it zooms (which you can change) and the way you draw first, constrain aka dimension, later.