I had the original apple case for ipad 1, the poetic slimline for ipad mini, smart case for ipad air, and the kavaj for ipad air. So for the air specifically, just the smart case and kavaj. Although I'm sure the poetic slimline for the mini is pretty similar to the slimline for the air. Overall, I like the kavaj best of all the air cases I've used. I'm just not a fan of the two-fold covers like the apple smart case and poetic slimline. If you like the two-fold thing, then I'd go for a poetic or any other similar cheap smart-cover clone. You really can't beat them for <$15. Smart case at $80 is just not worth it.
But, if you're like me, and prefer the original ipad case's one-fold cover design, then the kavaj is great. Overall I'm really happy with it, although it is not perfect.
As far as rating the smart case and slimline... I'd say the smart case gets a 5 for me. It's slim, light, The back is nice, but the cover sucks, and it's just way too expensive for what it is. I'd give the poetic slimline a 7 just because it's way less expensive.
I just want to reiterate that my ratings are heavily weighted by the cover design. If you don't like the one-fold design, the kavaj is not for you.