Pros
Large company, good for graduates to gain experience in a large company and learn the ropes of working with a major organisation. Good career prospects for early, mid level employees and training programmes. Good company benefits package.
Cons
Large can often mean confusing and because it is such a large company (and even larger with Three), it can take a while to understand who does what and where and who to connect with. There seems to be constant change (this happens a lot with mobile companies it seems ) . Company strategies change, senior people and groups of people seem to move around quite a lot so you might not be talking with the same peoplein 3 months or 6 months time as they switch people around quite a lot. Getting things through there systems takes a long time, seems there's no quick way around the system and lots of layers.