The attached image shows a comparison of the same scene lit in two different ways.
The image on the left was rendered with a sky dome of hemisphere lights and an area light casting shadows only. The image on the right was rendered using a combination of a small sky dome and ambient occlusion.
The image on the left is better - despite faking global illumination. It also rendered much faster than the ambient occlusion render. The whole point goes to show that "cheating" during the rendering process is the way to go!
You can click on the images to view them full size and compare the important differences between them.
