
Went of a few TfL buses on Saturday and I tested the mobile Countdown site quite a lot
http://m.countdown.tfl.gov.uk
I checked whether I could find the buses going the other way as well as the buses I observed while waiting at stops. I was very happy with the functionality and speed and ease of use, however it was clear that the system is failing to pick up many of the buses at the moment, which is to be expected as it is still being tested. So if it says the bus is in 2 mins then the bus will probably come soon, but if it says 20 mins that doesn't mean there won't be one in 10 minutes.
The system also covers TfL buses outside Greater London but only if the bus stop is maintained by TfL. For example most stops between Redhill and Croydon on the 405 but excluding Redhill Bus Station.
I would argue that there is little need for an app when this mobile site is so good, but I expect that the existing apps such as 'Next Buses' for iPhone/iPod Touch will incorporate the TfL data when it becomes available. The Android app 'TFL Live Departures' already incorporates a button for bus departures which says "This feature is coming soon".