Vantaggi
As a smaller company you do get to venture into areas that you may not be able to at a large publicly traded company. More responsibility via larger design projects and project management can be within your grasp sooner if you prove you're up to it.
Svantaggi
If you would like to take advantage of the early move into more responsibility (project management for example) make sure you do it because it interests you not because you're looking for a salary boost. Any new responsibility you take on will be spun to be a great opportunity for your career development instead of you providing more value to the company. Seems to me like everyone is rushed through the technical as fast as possible to become a project manager. The result is that the 5-15 year range has mostly become PM's and is not interested in banging out any technical work and sometime lacks the technical knowledge to support younger staff. Management seems to think that they can throw a body (any body) at a task and manage their way to success. The result is a lot of interns and entry level engineers, and very few mid level and upper level experienced engineers that still do technical work.