They're quite good fun. Buy a ticket which is good for 48 hours use. You can take any of the buses (they have several routes) get off whenever you want, and get back on any of the other buses going your way.
It's an expensive way of getting around, so don't use it as public transport, but for a view of the city's landmarks with entertaining commentary, it's fun.
You will be asked to tip the commentator. Not used to tipping, here's our little guide:
Phil at the safety hub. about 6 years ago