Pros
It's the scale of the company and the chance to get exposure to a true enterprise environment. Citi has a good depth and breath of on going projects and various teams. Also, there's the global reach of the firm which means you get the opportunity to work with teams in remote places. Given the emphasis on off shoring and compartmentalizing tasks working here also affords an opportunity to improve your communication skills and overall interaction skills which is a good skill to have in order to make the jump to the next step hopefully in another company. Also there's a lot of brand recognition for the company which is good if you're into that sort of thing.
Cons
The bureaucracy of the company. It is so large and bloated that there are layers upon layers of middle management. Middle management in particular is often fearful of making a decision so ultimately all decisions and actions of individuals in the company become a CYA mode decision. (hopefully everyone reading this knows what CYA means ... hint: cover your @..) all this means that most ppl here are very risk averse and more than that because of the office politics are always looking for a reason to really screw their fellow employee especially if said employee is on a different team. There are a lot of power struggles going on especially at the lower middle management levels.