Pros
Great people, great culture--really feels like family as other reviewers have mentioned. Good perks: free lunch on Thursdays; commutation, gym, passion pursuit reimbursement. Flexible work from home policy.
This is a great place and environment to learn. If you are in a media planning capacity, you'll wear many hats and touch every aspect of the accounts you are in, no matter what level (this can be a pro and a con, depending on how you feel about it since you might be tasked with "assistant-level" work as a senior planner or supervisor. There is a more clear separation of duties for AMDs and up.)
Good people is Generator's greatest asset and I believe that is the number one reason why people stay as long as they do. Greg Messerle, Generator's President, has his heart in the right place and is very accessible.
Cons
Low redundancy within teams, no clear maternity leave benefits, and not many training opportunities.