Pros
Twitter is probably one of the best place to work at. The people are great, people care about your well-being from day one and the perks are good. On the other hand, you are expected to work hard and be completely dedicated to your work. You will be given a lot and you are expected to give a lot back which makes perfect sense. New-new hire orientation is solid and really useful plus you will get to meet some employees from the early-days. Best way to see how it's like is to be invited by a friend or an acquaintance for lunch. (The food is great; there are all kind of foods)
Cons
There are definitely carrier opportunities however during the first week, I felt an employee needed to move mountains to be promoted. My point of view isn't set yet so I will see in the future.