Pros
Love the energy and the overall atmosphere of my colleagues. These folks are so upbeat, optimistic and passionate about the product it makes you want to invest your own money into the company. Senior management does a terrific job of talking about high-level goals and accomplishments on a bi-weekly basis. The vibe during those meetings is electrifying, thanks in part to the CEO Dick Costolo who's both energetic and hilarious. Ideas put forward by non-managers/VPs, including those from projects that happen during a quarterly hack week, are also taking seriously. There are forums where such ideas are discussed, and when appropriate they're adopted into the product. Last but not least, there are tons of amazing perks including 3x/day free meals prepared by a world class chef (seriously, you have to try it), free beer on tap, lots of activities/classes, etc.
Cons
Some people might be put off by the narrow pyramid of direct reports reaching up to the CEO, i.e. there are too many layers separating most employees from the CEO in comparison to some other companies.