Pros
My job is challenging to the point of not knowing whether I can succeed, which provides quite a thrill once you get used to it. Also, I get to work on truly ground-breaking ideas that I've never even thought about before joining the company. Finally, people listen to you here...no matter whether you've just joined and they're a few levels up the management chain, if your ideas are good they get attention.
Cons
The business is growing so fast it can be pretty hard to keep up with the things you need to know to do your job. Also, because we're so interested in technological innovation, it seems like we're constantly interested in a slightly better solution than the one we have (for a given internal need). These collude to produce an environment where documentation can sometimes be hard to find, and it can mean tracking down a specific person to get answers you need to do your work. Since we're a truly global business, that may mean dealing with awkward timezone differences to ask questions.