I have a Ristretto for 11" MacBook Air (there's a 13 too) with the Absolute strap and the Guardian Dual Function Light. I also added an extra 16 inch key strap. I've got the Cocoa/Cayenne color.
PROS: Very light, very easy to use, "decent" space to put stuff (but not by a wide margin as you may imagine. Very nice materials. The strap is very comfortable and given that a MBA11 is very light (1kg) the whole thing is very light (which is what I wanted).
CONS: Not a cheap thing. You can pay close to 200 u$s if you add stuff. Perhaps the only gripe I'd add is that the compartment where you put the computer could be 1/4 inch taller (yeah), because I doesn't have a locking mechanism, you just insert the computer in there and fold it in (then you close the bag of course). But the MBA11 barely fits (in height) in that compartment, making it hard to keep it closed. Hard to explain. It's not something too annoying but it happens. Maybe if the folding piece were a little bit larger, it would fit deeper and therefore this wouldn't be an issue. Take a look at picture "m" in the website and you will see what I'm talking about. That thing doesn't like staying in there :)
The nature of the bag also means that if you put too many things in there, although it will close and lock, one of the sides may not cover and the interior may be seen.
Would I recommend this?
Yes, it took me a while to find a specific MBA 11 bag (not just case, I didn't want a case). And this one coming from Tom Bihn (good reputation) was exactly what I was looking for.
Pricey if you add stuff, but you don't really have to.