I just took a Southwest Airlines flight that was wifi enabled. I couldn't resist the temptation to give the wifi a spin.
My review, in a nutshell? It costs $5, it appears to work with (at least) HTTP(S), Outlook, and ssh... but the performance leaves something to be desired.
The Wifi gateway also appears to mangle HTML passing through it in order to display a Southwest airlines banner over the top of the pages. This does provide some function -- it indicates your current altitude and ETA. But they could have included a way to disable the behavior.
The bottom line? It works, and that much is neat. But given the performance limitations (and keep in mind this is just one data point), I doubt its utility for anything but basic surfing. But on a long flight, I don't think books or magazines could make the time pass by with such ease.