Pros
Company encourages employees to find opportunities and take on responsibility. Very much of the "see something, say something" mindset. This gives younger employees a lot of responsibility and ownership. As discussed below, onboarding (fondly called "AppNexus University") is excellent. The perks are good (free food, basketball court). The people are fantastic.
Cons
Middle management is gravely inexperienced, and managers do not support internal transfers. Also, senior management is a clique. It's a great place to roll up your sleeves and get stuff done, but don't expect anyone to teach you best practices. Strategy is haphazard at best. I've been at the company for two years and have lived through no fewer than 5 reorgs. Also, HR needs to show some analytics capabilities; demonstrate how and why we do things.