Pros

There are lots of smart technical people. Business and Project Managers are more like average though.
The company behaves like a single global entity. There are no branches, there are only offices in different regions.
Very nice office with free beverages, food, videogames and books.
Most savvy people in software development know the company and that's good for your resume.

Cons

Likely to travel a lot and lose some money in the process and not everything will be expensed.
HR is extremely ineffective.
Resource Management process (allocation into projects) is just fire fighting. There is no plan and unless you're in good friends with senior management they don't know your skills or who you are.
The office may be nice but you're always on client side.
Managers are directly responsible for 100+ people and those people rely heavily on management for everything as they're on client side.
Account management is fire fighting as well.
Most good tech people are leaving and new hires are not filling the gap.

Advice to Senior Management

Acknowledge the problems. People are the only thing that makes ThoughtWorks any different (and I mean competitive advantage) but the lack of perspective is burning everyone. Forget silly HR schemes for review and "sponsorship". Wake up to the fact that flat hierarchies are great but can't scale. Wake up and realise that they're competing (for people) with Googles and startups, not IBM and EDS.

And stop using "that's how consulting works" as an excuse for everything.