I used to do that Airtran thing, but you have to be super flexible. One time, all flights were delayed and/or canceled because it was raining, so I was stuck in New York. The gate agent told me the Airtran U passengers get lowest standby priority and I might as well go home and try again the next day.

At any rate, I have bought regular flights the day of and it wasn't an impossible amount of money, but I doubt I'd do it again.