It's great to be at a company that values helping to grow their employees. Being a small company leaves a lot of room to gain new professional experiences and develop a lot of useful skills. A highly collaborative environment helps everyone learn from each other, even outside of one's own job function.
Plus there's always the classic and not so classic list of San Francisco perks: flexible work hours, food and drinks, free bike, unlimited vacation policy, memorable company trips, etc.
Inconvénients
If you prefer to be told what to do, no one's really going to tell you. Life at a startup can be chaotic, and you might not be happy if you'd prefer to focus on a specific set of responsibilities.
Awesome people, great environment, benefits are good with a unique bicycle allowance (which I love!), very flexible work schedules (they trust you!), collaborative, people work together and are helpful, best line, "it's all fixable".