Pros
The company seems to promise to grow your career fast (this can vary depending on where you're coming from), they usually offer higher salary/compensation than industry average (this varies depending on the field). Sometimes very intelligent, accomplished and "cool" people to work with, with interesting backgrounds, across a variety of fields. Insurance coverage is great. Exciting opportunity to be part of the creation of something new. You could definitely learn a lot very quickly.
Cons
This could depend on the team you're on, but it could be argued that employees end up not getting the big rewards they were once promised when they were first offered jobs. There tends to be a culture of very long hours, a lot is usually asked of employees, often working in small teams and having to work more to compensate for the small size of the team. It wasn't unusual to work 12-14 hour days sometimes, even more, including weekends. They seem to fire and hire new very quickly, (or some employees may choose to leave quickly) rather than to invest in their current employees. Things may have changed, but if you join, know that what is being promised to you may not be realistic and you may be disappointed with what you end up getting given how much may be asked of you.