I know exactly what AutoCAD as I am currently using it in my class to design Houses. So not exactly for just Mechanical Engineers but also Architects.actually it can be both, but it's not really a 3d GRAPHICS program... just a model design program for Mechanical Engineers
Soft Image XSI Advanced, its about a $5000 program, i obtained it for free by searching for hours, turns out it's way to complicated for beginners, but endless possibilities,
http://www.softimage.com/
I use Maya but my vote is for 3DSMAX :nuts: