They sucked basically. A bendy bus takes up twice as much space as a double decker while being half as fun to ride on. Even with double deckers many bus stops operate beyond capacity with multiple buses in contention at a stop, double deckers allow 3 buses to load simultaneously allowing six busloads of people to board in a space that would only allow for 1 bendy bus, essentially tripling the capacity.

They also cause problems for other road users such as cyclists.

Bendy buses work well if your city has wide straight roads. London's roads are neither.

On the other hand if you have trolleybuses (like in my city) the double-deckers are a problem and articulated buses aren't.