I have not been white water rafting since I was in Costa Rica in 2003. They were super crazy there and liked to flip you out of the boat on purpose. When the option to go rafting in Bali presented itself, it seemed like a great idea.
We booked through an agency that provided door to door service for everyone. It was very nice to not have to think about getting to a meeting place or getting to the rafting destination. The downside, random people can curl up next to you.
There are various companies that will take you down the Telaga Waja River and we went with a company called BMW. Because of the time of year, the river was very shallow and we got stuck a lot which sort of diminishes the thrill. However, the guide did his job and actually did most of the work so we could enjoy the beautiful views. There is a 4 meter drop at the end which was probably the most thrilling part of the entire trip and at first seemed dangerous, but they do it all the time and we had a blast!