I've yet to try out WiFi while flying but hope to later in the year. What I'm hearing from many people who have used it is echoed here in this announcement from Lufthansa, one of the first airlines to deploy the Boeing Connexion service.
From this PDF file it would appear Verizon, which has the old GTE Airfone business unit under their wings now, is getting ready to launch in flight WiFi service in the USA.
I can't wait to try it.