Tried Birdies while staying in Paris for about a week last October. It was so much fun, although depending on the distance you need to cover - not exactly the cheapest one. We actually brought our bicycles with us to move around Paris. Used them once, then tried Birdies just for fun - and that's it, we fell in love with them, bicycles were forgotten for the rest of our stay. Again, this is not one of the cheapest joys depending on the length of the ride, but for really short distances combined with lots of fun – it's worth it.
I think this has multiple purposes. For elders in the group who can’t keep up with the group an excellent way for them to enjoy the city view
Ask the 1000+ people who have been ticketed in Santa Monica & Venice in the past week.