Pros
Entrepreneurial environment, lots of opportunities, growing, dynamic company, active incentives to influence firm strategy with big potential upsides from a compensation and career advancement point of view, excellent leadership, great communication. They treat you like an adult. Stable, the finances are well managed - they really do practice what they preach. Long-term perspective. They have a good history of avoiding lay-offs due to financial or market conditions, but they are not afraid to eliminate positions for strategic reasons.
Cons
This is not a forgiving environment - you will be held accountable, and quickly. Help will not be offered except at the beginning - you will have to ask for help and asking for help can be perceived as a sign of weakness if you don't quickly improve. If you don't fit into what your partner expects, you will be unappreciated and under-compensated. Personal relationships matter, sometimes too much. You must manage your own career and pay attention to what is going on around you. The conservative business practices can feel stifling at times. The management structure and culture means that important decisions are often delayed until a consensus can be obtained.