Pros
I have been here for a couple years, and so far I'm very happy with my job. In Silicon Valley it's pretty common to go to work for people who don't have a good understanding of how to run a stable business and keep their employees happy. This company is an exception to that rule. In my time here I have never felt that I couldn't talk to management about something important. Working hours are reasonable, benefits are very good. Work/Life balance and career advancement are things in which management takes an active interest. I also have the ability to work on new technology and make contributions which make a real difference to our customers. I have no doubt at this point that coming to work here was the right choice for me.
Cons
We're rapidly outgrowing our current offices. This means the facilities are a pain point. This is a temporary situation though, and we'll have more room soon. This isn't much of a con for me, but we're still a startup, and that brings with it all the inherent risks that working for a startup entails. Since we're a startup, you're also expected to be capable of learning new things quickly, and to be proactive about identifying and solving problems without direction. Things also change very fast. These are mostly standard startup frustrations, and it shouldn't discourage you, but it's not for everyone.