Vantaggi
The perks are great (free beer on tap, free meals and snacks), the people are wonderful, and the work is exciting as heck. I'm always happy to go into work, and happy to be at work. I'm surrounded by incredibly intelligent people, working on exciting technologies, solving interesting scaling problems that nobody else has dealt with, and learning new things every day.
Svantaggi
HQ is located in a very sketchy neighborhood (Civic Center / Tenderloin), and I've often walked past heroin addicts shooting up on the sidewalk. There's plenty of homeless and desperately poor folks, but I'm not harassed by them. It's a little depressing, but Twitter makes a strong effort to encourage its employees to help out the locals in some small way. There is a certain amount of growing pains at Twitter, and some of us newer folks often ask ourselves "why did this get implemented this way", but that's common at any startup. Some of us from the big companies (Google, Yahoo, Facebook) know there's better ways to accomplish the same task, but that's why they hired us. There is also a certain amount of groupthink at Twitter, but I don't think that's any different from any large corporation.