Pros
Environment - you're not likely to find a small agency environment with as much energy as this one
Experience - the client roster is large and varied enough that no matter which department you're in, you'll get exposure to other teams, other initiatives and will be able to build your resume in no time
Balance - Combine standard PTO, monthly WFH days, sick days and Summer Fridays, in addition to flexible scheduling when you're in the office make it easy to take care of personal business and take those vacations you've been dreaming about
Job security - 451 doesn't hire based on clients, they hire based on need, which means that if they're hiring you, you'll have a job there as long as you want it. It also means that you'll be busy at work, but that's never a bad thing.
Other Pros: Proximity to public transit; Neighborhood; Experience of Senior Leadership; Pizza Fridays Pot Lucks and the Kegerator; Couches and bean bags; office dogs, office dogs and office dogs
Cons
Shortly before I left, the company took an employee survey which revealed a handful of concerns, and they'd put in steps to resolve those issues before I left, but it's too soon to see how well they're working. As such, I don't really feel comfortable posting cons that might have since been resolved.
I should also note that I left due to relocation, not anything related to the job or company itself.