This would normally have been the kind of thing I would bypass, as anything with the word "tour" tends to make me shiver, but bearing in mind the fact that myself and my companion had never been to Paris before, neither of us could speak French, and, as much as he'd like to think otherwise, our map reading skills aren't superb, we opted to use this service to get our bearings and find those must-see places.
The tickets cost 22€ each, and you can buy these on the bus or from many of the hotels around town. They allow you to use the bus for 2 days, and once you have paid for your ticket, you are issued with earphones so that you can choose whether or not you wish to learn a bit more about the city or if you wish to sit and take in the sights without commentary. I particularly liked the fact that the tour was not intrusive, and that we were free to come and go as we please, with no time restrictions on what we did.
The bus runs until about 6pm and takes a circuit of Paris, including the Eiffel Tower, Champs Elysees, Arc de Triumphe, Louvre, and Musee de Notre Dame, amongst other places. The only problem we found was that once on the circuit, we couldn't work out how to get off and get the right bus to take us back towards the station, so for about an hour we felt like we were living Groundhog Day. In the end, we gave up, walked back, and soon realised that once you get to know where you are going, it is a pleasure to walk around the city, too.