This app has been down for the last couple of weeks as O2 made some changes to it that conflicted with the application.
It has now been fixed and after checking my balance a pop up on my screen said to tell all friends that the app is free this weekend !
Deff worth the download, a great little app to check your balance and it was well worth the 59p yet alone the chance to get it for free this weekend!
EDIT: Just checked on my iTunes and it is already showing as FREE from today so download away!
Link updated direct to app in Itunes app store