Pros
Secure, stable, established. Would be a good place to try if you want to see what it's like in a really big bank. Lots of benefits such as food and beverage, special offers etc. afforded by the bulk of the employee population. Has very high level of brand-recognition at least in Hong Kong.
Cons
Probably too big to manage effectively. Everyone pretty much becomes a cog in the machine, unless you become senior management or happen to work in a fairly independent division / location. With such size, it's difficult to avoid bureaucracy. At least in Hong Kong, back-office functions can be so narrowly defined, while overseeing a complex range of customers, that no one really has the 'big picture'. There often is a disconnect between what top level management sees and what actually happens at the working level. e.g. organisation of divisions is viewed differently depending on the level it is viewed by.