Pros
There's the usual stuff (beer in the office, cool location, awesome coworkers, flexibility to work remotely, loads of vacation) of course. But that all kind of pales in comparison to the fact that the product is really, really awesome and the work I do is really interesting. Things have changed a lot, but I can say that I think that everything has been for the best. Personally, I found a niche that I could help grow, with the support of management, which has been a great experience.
Cons
Resources are limited (especially time) when a team is smaller, and sometimes it takes longer for management to get around to something you're waiting on, even if they hear you. In the past, it was an environment where things could change pretty quickly, but that's the nature of working for a start-up.