Pros
First and foremost is the excitement of building something that will make a difference. In my career, it's generally been easy to be proud of the work I've done, but being proud of what my company does has been less common. The opportunity to take everything I've learned and apply it in a way that helps myself, my co-workers, and our site's visitors is nice -- being recognized and appreciated for the work I do is even better. The team is passionate and respectful, which is a perfect combination for a happy and productive work environment.
Cons
There are the standard pains of building a brand new functional product (and everything to support it) under an accelerated schedule, but the biggest downside is the location -- I miss being able to ride the train to work every day.