Pros
It was an excellent environment for growth - lots of stuff is always happening and people are super excited to be doing it. There's a lot of belief in the mission and no time for BS. The engineers are all very talented people who could easily go to Google/Facebook/etc but choose to be here instead. The business people are also very smart and know how to interact with the engineers well. There's lots of room to make choices that would normally be made for you at a larger company.
Cons
It can get a little grueling at times - it _is_ a startup and they take agile culture / commitment seriously. Ultimately though it's not a "con" so much as a matter of personal fit; if you're looking for a job where you can take it easy and work from 9 to 5 so that "work stays at work," keep looking.