I have not worked for Thoughtworks, but I have interviewed for them twice.
For the first interview, the technical interview, P3 interview, Logic and Assessment test was really great in my opinion. I had two really strange experiences during the interview process. There is usually a brown bag round, where you attend a presentation from a Thoughtworker and there is a general discussion around the topic. Coincidentally, the presentation happened to be in the field of my thesis. I had good knowledge of the topics. To my utter disgust, everything that the presenter talked was mathematically and in terms of computer science wrong. I was shocked by the lack of research that a presenter can do on the topic. However, I am a strong believer in countering incorrect facts. I started to question the presenter slide by slide on everything mathematically which really irked the presenter. The point came where the presenter started claiming sorting is a NP-hard problem. I was shocked at the mediocrity of core conceptual knowledge veiled by smart branded technological-hipsters. Anyways, I went for the pair-programming round, which was again bad. The interviewer from the start was really condescending and attacking me. It became so chaotic that I couldn't produce 10 productive lines of code. All of this left in me, a terrible experience.
One year later, they came out with a great position and I thought maybe I should forget last year and apply again. I received a response within one day of application and went through the programming assignment round. They skipped the P3 interview and technical interview because apparently last year it was already quite good and they don't need to spend their time again on it. So, I went to the pair-programming round and it was fine. I was told that I will be informed by next monday (which was the next business day after Friday). I did not receive any response for a week, so I followed up with an email. I receive a response that I will be informed in a matter of few days (tomorrow was the exact word). After no information for another week, I followed up again and then received a response after 4 days, that they are undergoing internal-changes and it will take an indefinite time.
I am a bit unhappy with this kind of interview process, why waste weeks of my time and a day of my leave when they weren't even serious about hiring. Typically, first thing that the companies do when such changes happen is to freeze hiring and not waste other people's time.