The people, the opportunities and the benefits, not to mention being at the beginning of something with HUGE potential.
Cons
Like ANY start-up, you have to ask yourself are you willing to put in the 60+ hours a week, being "on call" at any time to help other in need of support, and you MUST have the ability to take initiative to get things done on your own and not look back. You'll make mistakes and you may pay for them, but that's life at a start-up.