Pros
Incredibly smart people, a really innovative product, and lots of interesting problems to solve.
There's a lot of good, new technology being used here, and a good opportunity to learn about new things on an almost constant basis.
Cons
It's a new company and new industry, so there is some chaos on occasion. It's a little hard to identify with the product on a personal level, since this is a SaaS product, not an end user product. You definitely have to be a self motivated person, since eveyone's so busy all the time. Small company = things change and break all the time.