Pros
You're surrounded by peers whose day to day effort is a constant reminder to step up your game. If you're at all attentive to what's going on around you, you can't help but be driven to improvement. This is not the place where you cruise along at the top of the pile. The goals, the excitement, the accountability ... they're all shared experiences among the team. While the effort to reach each milestone towards the company mission is tough, we're actually making progress faster than any other competitor in the market. The compensation is reasonable and the stock options are awesome. Highly rated employees (annual reviews) are often awarded additional stock options for their efforts.
Cons
There isn't a career path that you don't create yourself. So, for many that doesn't sound very appealing. However, think it over. As much of a con that you may consider it, you're also in control. You can easily convert this con into a pro. Work life balance truly sucks. If you've got a significant other, you'd better make sure that they are on-board with this job as it's going to be a time suck. You simply cannot put yourself on cruise control. Our goals are not insignificant, so the majority of them are not yet realized. This means that every success is followed by the next need on the path to achieving the company mission. Get used to that idea and prepare yourself to look at a series of achievement on the way to an over-arching goal.