Pros
The company has a big commitment to career development. It's a young and fast-growing company, so if you show aptitude and ambition, you'll be given an opportunity to grow upward and take on greater responsibility. You're going to be a lot less held back by your job history than at other companies. Like all tech startups, everyone's under pressure and there are some tough deliverables, but the management team is serious about work-life balance. There might be the occasional push to get a big deliverable out, but it's not an everyday slog. There's no pressure to watch for the boss to leave so you can finally leave your desk and go home.
Cons
There's no way to get away with just coasting and not doing your work. Just as they're quick to promote someone who's doing a good job, people who aren't pulling their weight are out quickly. But that's also a plus, because it means that you don't have to put up with other people's dead weight. It's not a low-stress environment, and there's not as much structure as there is in a more mature company. You've got to be able to think on your feet, and the job you're doing is really going to matter.