It'll be interesting to see who jumps into this market!
I've been using the same Apple leather Smart Cover (first on my iPad 2, then on my iPad 3) for the last 2.5 years (if I did the math right). It's handled the wear very well.
I'm using an Apple poly Smart Cover for my Air, and I really like it. It's noticeably lighter than the leather Smart Cover, but I guess that's not a totally fair test since the Air cover has 9% less surface area than the iPad 3 cover I'm comparing it against.