Pros
World class engineers and a culture of continuous learning. There were reading groups, Rock Band / Halo / foosball / frisbee games, beer, and free food available almost every single day. We were given a full week free just to work on hacks of our own design and present them at the end. Great place to become really good at web development and make good money while you're at it. The recruiters at the other big tech companies know about TripAdvisor and their engineering standards.
Cons
Your experience can differ a fair deal depending on which team you intern with. Travel time to and from Newton can cut into your free time if you live in the city. The shuttles to and from Cambridge help a lot, though. Using a 30" monitor all day makes your computer monitor at home feel small.