Pros
The company is full of hard-working, intelligent people with a great sense of camaraderie. In general, people will pull together as a team to get something done, regardless of title or rank. With a start-up atmosphere, there is the flexibility and opportunity to create your own processes and define your position. If the work is there, you're welcome to speak up and pitch in to get the experience. Social atmosphere is very strong with many people grabbing drinks, traveling, and becoming friends outside of work.
Cons
Because of the start-up atmosphere, there is very little to know process - when there is, it's created by individuals in the respective position and difficult to implement company-wide, and open to changes should someone leave, get promoted, change positions, etc. There is also no official training period or method; being taught about the company and your job is at your manager's or your individual discretion. Basically, sink or swim. While great ideas are proposed, they are extremely difficult to execute due to the licensing and approval structure. Approval time far exceeds production time, resulting in very short turnaround for projects. Additionally, most people do not plan ahead or consider how last-minute requests impact other departments, work quality, or budgets.