I've used this a few times and never had a problem with availability. Mainly on European flights, but some international.

The only catch (but its not much of a catch) is that the ticket must be full fare. In practice I've found this to mean 'the most expensive fare advertised for the particular route'... i.e. you wont get D class (discount business) prices. Its the fares that are advertised as J1 or C1. Even so, discount J/F tickets are very rarely (if not never) less than 50% of full fare so you are still saving money, you just arent saving 100%. Bet on saving around 1/3 instead of 1/2 and you wont be disappointed.

dnw