Good if you have nothing to loose, want to travel or you're a junior dev
Pros
ThoughtWorks still has a lot of great and smart people really interested in helping you (specially for women). Depending on the project and client you land on, you'll have a chance to expand your network, experiment working with different industries and technologies at diverse locations across UK (and internationally). Also you'll get to eat for free (of course, it depends where you're working but if you're a consultant out of your usual commute, you can get reimbursed). There is a personal development budget that you can use to get some training and go to conferences. A few interesting events if you are attached to the London office (don't know about Manchester).
Cons
Where do I begin? I've joined as a lateral hire (in TW lingo, this means someone that already had a career before) and spent almost a year trying to understand how TW works and what's the purpose of it all. Is it a non-profit disguised as a software consultancy? Body-shop that sucks less? Roy's little experiment? I left without any clue. First, as a consultant, even if they tell you that there is no manager, you'll be at the mercy of the staffing team, who can then send you to a client/project that can be great or just a body shop. TW has a great office in central London but don't believe the hype. If you are a consultant, you'll be spending most of your time at a client site, dealing with legacy tech, boring people, enterprise culture and usually travelling to less desirable locations across the UK. Your ability to grow and work on interesting things will depend on how good you are in networking and getting noticed (ie, politics). Best projects are kept for the most politically skilled (sorry, introverts). Second, everyone likes to blast how diverse and inclusive TW is and how everyone is able to freely have a discussion up to the CEO but this is only true if you are on the "right" side. TW has a lot of wonderful people, but it also has marxists, social justice warriors, hardcore feminists, LGBT advocates (they are the ones dictating the agenda). If you don't buy into this, you'll have a hard time there. I was more comfortable at a client side than in the London office (I felt no connection with other people there). And the idea to choose a sponsor to help you advance in your career is a joke.