Pros
You learn A LOT - Startups place loads of responsibility on their employees. They'll hire you because of your skills, but founders expected much more. You help with everything at a startup. Often, it's work outside your job description, so opportunities for learning and growth abound. Founders and employees work together; there's no middle management, so you learn from the best. Employees work without supervision. They make smart decisions and take responsibility for the consequences. The chance to steer progress motivates them to perform well. You can innovate - Startups need to grow fast. If they can't keep up in the fast lane, they'll crash out. Employees have the license to show off their brilliance. They deliver results with fresh designs and new concepts that capture consumer interest.
Cons
Nothing to say till now.