Pros
Fun and exciting company. Being a "start up" company, there are loads and loads of opportunities to "create your own niche". Even though you might have been hired to do "X", if you've got some great ideas, they are welcomed, and encouranged, and often times, tried. People are fun and the company as a whole is a lot of fun, with weekly meetings and recognitions, pot-luck parties, etc. It's a laid back, casual, and great place to work.
Cons
As with a lot of companies, I saw several people get promoted because of either who they knew or how well they performed in their current job (which, when promoted to, for example, a manager, they might not have been well suited for a management role). There are lots of "hanger's-on" who have been in their role for a very long time, but I'm not really sure of what they did or how it fit in with the corporate goals.