Pros
The best thing: a shiny Italian coffee machine
When I joined, there was lots of structures to be build. I was part of introducing scrum, continuous integration and docker, so there were many things to learn. It was very cool implementing different ideas without too much constraints on time and budget. Senior management (well, the CTO then) was always up for a discussion about any topic.
Cons
Here is what went wrong: The company hired too many juniors much too quick and the open atmosphere became a burden: people joined the discussion table that had so little experience, everywhere else they would have never gotten no responsibility whatsoever. At atfinity though, they kinda got a chance, but ultimately ruined it for seniors like me. Bugs from wrong decisions bubbles up more and more and it when I left it was already clear the team. Reds to be changed. I know the founders quite well and I know the meant well, but, honestly, the whole team needed to be replaced. And that was 2018, I heard it took many fights and tears till that finally happened in 2019.
I heard now in 2020 the company has changed a lot and is doing pretty well. I still have, very sporadic, contact to the founders, so I know the CTO is again in charge of the code. He is one of the best coders i have ever met, so quality probably improved significantly.