And to add to what Dave posted, on the last 3 cruises I have been on they have had a "first day special" where if you buy a 100 minute package they gave you free minutes (I think it was 50 minutes free but I can't remember for sure.) So if you think you will want to use it a lot (for emailing home or keeping in touch or gasp... working) then check in the cafe' the first day and see if they are running a special.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007