Duolingo

• Generally a joy to show up to work every day.
• Excellent engineering culture. Everyone I worked with seemed to be genuinely excited to be there, solving problems and working on new things.
• Management highly values employee satisfaction and career growth. I had lots of opportunities to learn new skills and take on new challenges.

• Living in Pittsburgh isn't for everyone, and most positions are located there.

• The company has grown a lot over the last few years, which has made for constant change in teams, people, processes, features, code/architecture – just about everything.

This isn't really a con for the right type of person, and it may mellow out as the company transitions out of the late-stage startup phase, but be prepared for a very dynamic environment.