Pros
Working on a brand-new product, so it was almost all new development as opposed to maintenance and bug fixing other people's code. What I especially enjoyed was that as opposed to working on little things here and there without a chance to see the big picture (well, some of that too, but no more than normal), developers were given sizable features to work on and own. It was nice to be able to build features from start to finish. These features started with a tech doc/BRD so didn't have to do the requirements piece of it, and they had a good web team so I didn't have to worry about it making it pretty.
Overall, one of my best jobs. It was nice to be treated as a professional, trusted to write meaningful code, and not micro-managed.
Cons
Around releases it can get rather intense- lot of code to crank out/polish up quickly. Think that was mostly due to it being a new product- things seemed to be hitting a good sustainable pace when I left.