Pros
Lots of room for growth and advancement. You can really pave your own way, and make the position your own. Very flexible work environment. The culture is fun and engaging - everyone wants to learn and share their knowledge. Exposure to many different technologies and hands on experience makes it a great place to learn and grow quickly.
Cons
Fast paced, and not a lot of micro-management or hand holding. This can be stressful for juniors, but recent additions to the team contribute a lot. A really small company means you'll have to wear lots of hats and step outside your comfort zone. This is a con if there are tasks and work you think you've "outgrown" or dislike.