Pros
Collaborative, progressive culture. Opportunity to make things happen, and to have your voice heard. Leadership is transparent and communicates fearlessly with employees. Benefits are great, from excellent healthcare (for you and your family or +1) to perks like fitness reimbursements. Twitter has excellent training and learning opportunities, helping professionals continue to grow in their skill sets and discover new areas of interest. Additionally, I believe that Twitter is doing great things in this world: liberating voices, assisting in the igniting of revolutions, and bringing light to things that matter around the world.
Cons
$401k plan could be better, but Twitter is identifying and seeking to resolve this. Pay should be more competitive.