Reviews on amazon:
"Here's the catch:
I've gotten free flight certificates from JetBlue before. They are NOT accepted on every seat/flight. Only for flights that they decide. So, if you see there are seats available on a flight from New York to Florida on Friday night, and you call them, they say "No, sorry, the voucher can't be used on that flight, you can only use it on next Tuesday's flight" or some other nonsense. There have been news stories about people who couldn't make good use of their free flight vouchers."
"You can only book flights on fridays though."