Pros
Twitter is a small, fast-growing company with a management team that has worked together in the past. There is always way too much to do on any given day, which allows for specializations beyond a given job function. Software engineers enjoy many new technologies such as Scala and real-time consumer messaging while operating in a small team with the capacity for big impact. Many employees are ex-Googlers with experience in large-scale systems yet eager to escape bureaucracy. Twitter employees have passions outside of work, which makes for more interesting coworkers.
Cons
A fast-moving startup where it's sometimes easier to see what isn't getting done than reward the good efforts being made by staff every day. Everyone is stretched so thin that sometimes the pressure takes over.