Pros
Great people, fun culture, good perks & benefits - health & dental are excellent compared to other companies especially for families. Wide open opportunities for advancement or different career paths. People are constantly changing departments with no opposition from managers - just need to see something you want to do and ask for it. If you're doing a good job, chances are you will get it. So from that standpoint, you're able to get so much more experience than you would at a more hierarchical or tenure-based organization. Pace is really fast. Because of that, employees are empowered to make decisions & run with them. Scary but exciting. And lastly with a company growing this fast, the equity grants are a nice carrot.
Cons
Work life balance can tilt askew and sometimes you just have to recognize that you can't do it all. Often you're loaded up with much more than you can reasonably accomplish so you have to decide what's going to slip. Salaries are not terrific, especially for entry level. You're in it for the start up experience and the camaradarie, (plus the quarterly parties & monthly team drinks) not the salary. We need better career path planning and more concrete review criteria, but I hear some of that's in the works. The problem is, roles are constantly being created and changed, new groups are forming and so by the time you define a career progression within a department, it's obsolete.