Pros
Fast-moving tech startup, where I've worked with some of the smartest and most hard-working individuals in the world. It's great to be able to working amongst people who have previously built and scaled companies such as Twitter, Facebook, Netflix and Google. The people & environment are very friendly, and make it a real pleasure to go to work every day. Everyone also has a very healthy attitude towards working from home and flexible working hours. We often have team nights out and drinks after work. No expense is spared on learning materials, bring influential speakers into the office and generally developing one's career. There is a great culture of open and honest feedback, with a real emphasis on improving process so that problems of the past never happen again. This was eye-opening for me, coming to the company as you can see how much more efficient and politics-free everything becomes when everything is out in the open. Free food on Fridays :)
Cons
Honestly, not many but here's a few small ones: The company has grown so rapidly that there sometimes seem to be basic things which should have been fixed long ago, but no-one has got round to it yet. Much of the core business still depends on an large monolith application (which is slowly being de-composed) but this does cause friction sometimes when wanting to move quickly and ship a feature. I have been lucky to not have to work regularly within the monolith. We care constantly hiring new people, which is great, but this means it sometimes becomes harder to keep tabs on everything that is going on and what everyone is working on, as you would do in a smaller startup.