Just one is just designed to be fun--it's not super hard and you can screw up a lot and still get a decent time. It has a sprinkling of a lot of different spices...wallrides, jumps, tight sections, tricky turns, dirt, wide drifting turns, and so on.
It's probably the most purely enjoyable track to drive that I've released in a while...but I'm biased.
Any feedback is appreciated, and replays are especially cool. I'm sure that the AT could be quickly beaten by any of the really talented drivers out there, and I'd love to see a replay!