Pros
I worked with some great and even a few exceptional people, learned a lot, had some fun times, some nice benefits, great way to see/hear about different areas of the software business.
Cons
It can be vaguely cultish, you are pretty much expected to LOVE Thoughtworks and be 100% on board with their way of doing things. Especially when it comes to Agile ideas, lots of dogma here. They are constantly pushing how great it is to work at TW, this really bugged me after a few months. Lots of corporate politics, they claim they are above them but in some ways they are as bad or worse than their corporate clients. There are also quite a few people around who have been with the company for a long time and form an 'in crowd', how well you do in the company has a lot to do with how how good your relationship is with those people. This can mean the difference between getting a raise, promotion, good work assignment or not. Feels very Frat house like and not in a good way. Thoughtworks markets themselves both externally and internally as being an elite company with top developers. As a result of this the are quite a few people with very inflated egos who make sure everyone else knows how great they are. If you ask the wrong question or like the 'wrong' technology they will belittle you. Think obnoxious people on forums, IRC, etc, it can be bad. The pay is not that great, they expect you to work longs hours and go over and above for the company without compensation. Getting stuck on a weekly flight commute sucks, a 4 hour flight every Monday at 6 am and Thursday at 9 pm gets miserable fast. The travel can be fun but few people enjoy it for more than 6 months to a year. Developer turnover is high, a lot of unhappy people, I have never worked for a company with such a high level of employee misery. There is a weird contrast where you have people who are very positive and then people who are miserable and sometimes the same person will display both depending on the situation. That can happen anywhere but at TW it is on a whole new level and comes with the consulting territory I guess.