I have, which model do you own? On my MBA, Killing Floor ran smooth-ish. A couple graphic intensive maps really overloaded it, with rain/fire or whenever firebugs or spam demos, my framerates dropped pretty significantly. Don't expect to play it at normal settings, all my graphics settings were around medium to low and I had to lessen the resolution. You'll have to play around with the settings to see where you'll be comfortable playing at.
I had a 13" i5 @ 1.7GHz and 4GB Ram (sig is old and needs updating)