Avantages
Absolutely flat organization does not get in the way of getting you job done. Learning new things is encouraged, supported and the most importantly, actually resourced. The CEO is absolutely awesome leader and lovely person. You get to decide how, when and where you work and you don't get crappy tools.
Inconvénients
Teams are mostly really small (1-2 people) and projects pretty demanding. You need to go and find the support (should you need some) outside of your team. And the salary is not exactly stellar.