Pros
Amazing developers work there. Many of the best in the field - and a real passion for learning. The investment days are rad. I made a few react apps, learned react native, and worked on marketing sites. Some days the designers would just draw! You don't have to worry about anything other than doing your job - the benefits are top notch and the office manager when I was there was incredible. I felt like I always had someone looking out for me because they weren't always striving to improve the office life.
Cons
Thoughtbot is still heavily recognized as a dev shop and we were working on changing that image. As a consultancy sometimes you take on projects you don't like. Many times that won't be the case, but usually 1 a year for someone isn't a great project. Usually that concern is voiced and if voiced is addressed.