Pros
People - Good teams and friendly people. Pet friendly. Flexible work schedule. No dress code. Slurpee machine. Basically all the little things that don't really matter in your career but help you get through the day. Big brands so it looks good on a resume, especially if you are on the account side. Good opportunity for title increases - but salary increase do not match (see below).
Cons
Tough deadlines - emphasis on getting work done on time, not getting work done right. Very little forecasting so 90% of projects are reacted to rather than planned ahead for. Team structures are not built properly. Some teams are very top heavy, some are very bottom heavy. There needs to be a better spread of thinkers vs. workers. Feels like much of account service has mentality that they work for the client rather than for imc2. Very little portfolio quality creative opportunities outside of new business pitches - resume looks good, portfolio book = not so much. Salary increases are not aligned with promotion title increases. If you are promoted from within you are given a raise based on your previous salary (usually 10%) rather than the salary range that new employees for the same position are offered. So if you are coming in and plan to stay longer than 1 year, ask for a larger salary than you would normally or you will end up underpaid once you've earned and received a promotion.