Pros
It's very rewarding to work for a company that contributes so much to the Open Source/Free Software community. *Every* product they start themselves or acquire is open sourced. There is a lot of talent in the company. My managers' styles have been very 'hands-off'. If you show you know what you are doing, they'll generally just point you at a problem and expect that you'll solve it. Very many of the development jobs allow telecommuting.
Cons
Many products have very geographically distributed teams. Time zone differences can make it hard to connect with the whole team. It also means there can be quite a few meetings. I cannot even begin to convey how much email there is. With many thousands of employees in 85+ offices in 35+ countries, Red Hat is a sizable company with a decent amount a bureaucracy.