I have a gigabit Airport Extreme at home, post March 5 firmware update, it works perfectly with my iMac and MacBook Air over WiFi.
My friend has a pre-gigabit Airport Extreme at home, post March 5 firmware update, and while it works fine on his computers and MacBook, it constantly drops and re-acquires WiFi, making it slow and frustrating.
Apple Discussions show I'm not alone, but don't show any good solutions yet
Apple - Support - Discussions - MacBook Air + Airport Extreme ...