1. AC6 is way different than AC8. There have been some good changes since then. It just seems to keep getting better.
2. The more training you get the better. AC can do a lot if you know how to use every bell and whistles.
3. There are some collages that are teaching AC to students. I reasonedly graduated from California State University Chico (CSUC) where I was first introduced to AC. I now use it in work every day. There is a web site that tells you where AC is being used, but I don't remember the address.
4. Like David Larrew said, "It is hard to compare "apples to oranges". " Other people in my office use AutoDesk, they seem to have a harder time making 3D buildings than I do. They say it's more work.
5. AC can do great renderings but only to a point. To get to that next level you will need another program, like the one's you mentioned.
6. AC has a large library of GDL objects plus, if AC doesn't have it you could check
www.objectsonline.com to see if they have it. What I've read, AC8 is supposed to have a easier GDL interface than previous programs. I don't know a hole lot about GDL so I don't use it.