Latitude passes are quite good, especially if you are SE with Concierge access (same day change anyone?). The only fees you will encounter are if you miss a flight, or don't use all the credits on time (extension fee, or just expiring credits).

Availability is mostly based on fare class. Sometimes there are issues with the flight pass system not showing availability that should be there...usually this is solved by calling reservations who will sometimes need to talk to the "resource desk." Personally I find that annoying because it can take as long as 30 mins to resolve.