Pros
Great to be involved in a business that has a tangible positive benefit. Decision making can be pretty quick- I was lucky to avoid the "death by committee" experience I've seen at other companies. Working closely with people who founded the company was a real learning experience- viewing business through the lens of an entrepreneur is vastly different to the perspective of a senior corporate manager. My experience was that my manager listened to my career ambitions and did his best to advance me. You do need to be proactive as career management isn't great but, if you are, then things happen!
Cons
The flipside of my last "pro" above is that these entrepreneurs demand a huge level of commitment from themselves, but also from those working with them. You need to be pretty firm to ensure a work life balance. As with other owner led businesses, things can get quite political. I sensed a lot of jockeying for position at times with people rising and falling based on political favour rather than ability.