Not living in a big city, we don't use them that often. However we used taxis a lot in europe when we lived there and flew all over the place. Usually a taxi to and from the hotel. London Black Cabs are great, because you can actually leave the child in the stroller in the his car seat. You can usually do this as well on a shuttle bus. Just put the brake on and hold on tight.

We definitely saw other couples in Europe who did not travel with car seats in cabs.

That being said, while in NYC I saw MANY instances of moms and dads just hailing a cab with baby and stroller and no car seat. I think they are all just so used to it. Some cabs actually have a flip down middle seat that is a car seat for the cause.