Pros
Ability to work for big name clients, and see how many top tier firms operate. Paid l&d time between engagements in the form of beach time. Product focused mindset of all TW staff provides excellent colleagues and working environment. Not sure if it still exists but the coaching program was incredibly helpful. Thoughtworks itself is a well known and respected name in the industry.
Kontras
They talked a big game about doing product development wholistically with all in house teams but all my engagements were getting seconded to other firms. Flipping between different contract teams can eventually wear on you. Leaving my third team just as we were getting into an amazing rhythm because TW lost their overall contract with the client was a little heartbreaking. Salaries were low compared to startups, but conversely stability and free time was much higher.