Pros
There aren't too many rules or established ways of doing things so if you're the type who wants to try out new things, write new code or that kind of thing you'd love it. You wont hear No as long as it doesn't cost the company money. If it doesnt cost any money then you can do virtually anything you want. Probably half the cool stuff on our site comes from ideas people had and tried out on their own. It is definitely a good place for people without much else going on in their lives because your not going to have time for any thing else.
Cons
The hours are very long. Expect 12 hour days at a minimum and you will work every weekend, except they give everybody a laptop so you can work at home so some of that 12 hours is at home at night too. Everybody has a blackberry because its expected that you be available at all times, even when you're on vacation. If you want a work life balance, start running in the other direction. Its basically an environment of work the person as hard as you can as long as you can, and when they collapse and die, people say they just didn't have what it takes.