I have tried 3 so far
First of all, I normally need to bring more than my 13" MBA so I usually need a bigger bag than one that accommodate the MBA alone. I use a sleeve with a backpack that has a large hydration pocket. Laptop backpacks will work too, but they tend to be too boring for my taste. Below are my short reviews of the sleeves I tried.
1. Hammerhead case
http://hammerheadcase.com/_/img/products/sleeves/macbookair13.jpg
It is a really nice case. There is a bit of padding, plus the leather, so my guess is that it's enough for most situations because the MBA is pretty solid to start with.
2. Incase neoprene sleeve
http://store.apple.com/us/product/H5197ZM/A/Incase_Neoprene_Slim_Sleeve?mco=MTY3ODQ5OTY
It's a decent neoprene sleeve which costs almost as much as the Hammerhead leather case but doesn't look as good in my opinion. I am not sure if it is any more protective than the Hammerhead case. Neoprene feels soft and compresses so easily that I am doubtful as how much shock the material would absorbs. But again the MBA is pretty solid so unless you are going to throw your pack around a lot, I suspect it would survive. I picked it up from the store because it is a vertical sleeve which is simpler to use in the hydration pocket of my backpacks. The pullover top does take a bit more effort but eliminates the need for a zipper.
3. Be-ez LA robe Air 13
http://www.be-ez.com/la-robe-air-black-addict.html
This is by far the most protective sleeve of the 3. It appears to be just slightly thicker than the Incase neoprene sleeve, but is a lot more protective I think. Unlike neoprene, the memory foam stuff that it's made out of is pretty light yet appears to be able to absorb more shock at least several times better than the neoprene material for the same thickness. I could tell by putting my hand inside it and pounding on it. There's a muffled feel as the hand is hit through the bag, unlike the neoprene where you can feel most of the force. the Be-ez also looks better in my opinion: somewhat like neoprene, but much denser in appearance. I got the Be-ez last because I wasn't sure I wanted the zipper top, but now that I tried it out, I prefer the zipper to the pullover top of the Incase.
I am going to keep the Be-ez as it is nicer and more protective than the Incase. I am undecided on the Hammerhead. While it is really nice, it's not as protective as the Be-ez, and the side enclosure means I have to remove it from my backpack to get to the MBA. It would work better in a messenger bag.
The bottom line is, if you want protection, I can't think of a better sleeve than the Be-ez La robe. On the other hand, the MBA feels pretty solid, so if you are reasonably careful, any added protection will do I think. So if you like leather, a leather sleeve will probably be enough. On the other hand, if you decide to go with a neoprene type sleeve, I think the Be-ez la robe is cheaper than most name brand neoprene case, looks better, and is many times as protective from my fist pounding experiment
