Pros
There are many diverse projects and technologies, which is understandable given the size of the organization. You can be involved in as many technologies as time and interest permit. Even with the turmoil in the financial sector, Citi appears to be fairly stable internally and there is always enough work to keep you busy and engaged. The organizational structure in the Technology side takes a little bit of work to understand, but does make sense. As a contractor, Citi is not in the least stingy in the hours they'll let you work. I have also heard that their alternate work location program allows people to not have to work our of a Citi office, I even know people who work remotely full-time.
Cons
There's a lot of work, and I mean a lot. This is ok for contractors, but it has to be hard for employees as they usually don't get comp time or overtime for working more than regular hours. The sheer size of the technology groups can make doing anything seem very slow and like your always getting bogged down in a bureaucracy that doesn't always make sense. Also, as Citi likes to buy other companies, we are always working on integrating acquisitions, so if you expect either things to stay the same or to be able to do things quickly, you're going to have manage your frustration.