We've used Big Bus happily in San Francisco and New York. We bought the 2 day ticket with River Cruise. There are at least 4 open-top bus operators in Paris. Here are our comments:
* By comparison with the other operators, Big Bus is more crowded and you sometimes can't get a seat upstairs.
* Big Bus is more expensive than some of the other operators.
* They all go roughly the same routes, to the same spots.
* Big Bus buses spend a lot of time sitting at the bus stops, even when there seems to be no reason not to leave. As a result, the lap around Paris takes an unnecessarily long time, and even just going a few stops takes forever.
* The commentary is pretty basic, although it does synchronize nicely with your location. There are long periods of time when they are playing music to you, which is a waste, considering that the time could be spent telling you interesting facts about Paris.
* The Montmatre loop is a waste of time. There isn't anything to see that you don't see elsewhere, except for the drive along Bvd de Rochechouart / Montmatre and Sacre-Coeur itself. Their timetable resulted in huge waiting (wasting) times. Your best option is to get the Metro up there.
* River cruise is great but you can buy one of many cruises a number of other ways.
Check out the other operators before you choose Big Bus. Take a look at Open Tour and Foxity and compare.
Buy your tickets on line, not at the bus stop. You get discounts on line.
Don't bother with the Montmatre loop.
Consider the hop-on-hop-off boat on the Seine, a better way to avoid Paris traffic snarls.