I have had my Birdy Anthracite fold-up bike since 2007. A Tour de France competitor built it and I bought it from him. Fold-ups bikes allow you to be more flexible and agile with your travel. For example: you can take it on a bus and then ride somewhere. The unique thing about Birdy fold-up bikes is the engineering, unlike others it folds at the wheels instead of the middle of the frame making it more sturdy. It is light and stealthy, it feels like you can nearly do instant 90 degree turns on it and I have a backpack that also unfolds into a bigger bag to fit the bike when I fold it.
The amount of times I have cycled somewhere and then for some reason needed a lift back or to go somewhere else in a vehicle, the ease of just folding it and being able to fit it into most spaces is invaluable. When I lived in Canberra I loved cycling to the airport folding it, putting it on the plane and then unfolding it in Sydney and continue riding and vice versa. It has lived in Cambodia. Now is a good bike in Brisbane.