Pros
Good solid brand, corporate reputation helps with clients. Quality staff across the organisation in varied fields and parts of the UK.
Cons
Certain teams are notoriously unpleasant to work in, especially within PDR (planning, dev & regen). There is no guarantee of senior support for or loyalty to junior staff. The sense is that some people will cope with that, others will self-eject.
Collaboration is almost unheard of. Expect to see fiefdoms, turf wars, different teams competing for the same work, personal conflict, people who've never spoken to colleagues etc. Asking colleagues to tell you about what their projects involve can result in vagueness and suspicion.
I was there over the recession and there were almost weekly redundancies, without explanation or any sense of strategy. Managers seem to have no natural instinct for anything other than project management and sales, and evidently they get no training in leading actual human beings.