Essentially your iPad 3G service is the equivalent of having your cell phone to use. Connect to the web, stream a movie, download a few books for the ride. Do what ever you want!
Has nothing to do with any device and everything to do with where the bus goes. If it remains in an area with adequate 3G coverage then it will work. To give you a poor example, I take the train to work everyday (40 minute ride). My iPad has great 3G reception for about 37 minutes of that ride. There's a dead zone of 3G in a certain area that's due to AT&T's spotty coverage.