Pros
Extremely talented co-workers to learn from. Very effective hiring policies is maintaining that. Super-energetic environment, can-do culture, high level of trust, opportunities to go and build something useful outside your normal work stream. Company's strong values got it this far; strong focus on maintaining and evolving an effective culture despite rapid hiring. Great level of passion and enthusiasm for what we build and what we deliver to our customers - it's useful, worthwhile software that makes a difference to our customer's ability to get things done, and we don't forget that! Strong technical and quality focus. We don't tend to get stuck on doing things the same way just because that's how it's been in the past; new hires have a good chance of bringing new knowledge/ideas and changing things.
Cons
It's chaotic, both technically and organisationally, as it deals with aggressive modernisation of legacy platforms and architectures into a more modern approach, and grows and evolves from a high-individual-freedom low-oversight small-company culture to one with a little bit more forethought and management that is able to scale. So far the balance is working very well though. Arriving as a new developer working on any of the main products (vs new infrastructure, which is different), there's a lot of complexity and legacyness, large teams and an awful lot to learn, both technically and organisationally, and this can be overwhelming - expect 6-9 months before you feel able to effectively contribute to gnarly stuff to the large products. You'll need to have initiative and be willing to reach out, ask questions and make things happen.