Until really recently I thought one of the stupidest features on a camera was auto ISO. Well after getting the Nikon D5100 and getting used to it a bit I have become a convert. I love the auto ISO feature. Basically you set what the maximum ISO is that you can live with (I go with 6400), set what shutter speed you don't want the camera to go below, set your base ISO (I always set 100) and then let the camera at it in aperture priority mode. As you shoot then if the light is so low that you cannot do your minimum shutter speed at the current low ISO the camera will start to ramp up the ISO to get the proper exposure. So if you set say 1/250th of a second and you need another stop of light the camera will bump up ISO to 200. So it is giving you the lowest ISO possible for the aperture and shutter speed.
This is awesome for a number of shooting environments, but probably rocks the most on simple day trips. I was on a vacation this summer and did not have to fuss with the camera settings while out and about sightseeing. I knew that the camera would handle all of that and I would get all my shots. Give it a try and see if you don't agree that it is really awesome.