I've been playing around with the free version of SketchUp. I think it lives up to it's billing as a very user-friendly 3D software package. I have used AutoCad in the past but it has been a long time so I'm familiar with some of the concepts. It took me about 3 hours to put together a model of frame for a small building.
There are inexpensive books and eBooks that make it a little easier to understand the concepts and give some tips that save a tremendous amount of time. I don't recommend trying to use this without a real mouse and a keyboard.