Pros
As with many startups, you can find yourself with a high degree of ownership and autonomy. People make space for subject matter experts to come in and run things the way they want. There's not a lot of bureaucracy or layers of approvals. The leadership team is made up of very smart, level-headed people. The HR team does a great job for a startup: for example, competitive pay and benefits, many team building events, 2x annual offsites, etc.
Cons
As with many startups, goals and initiatives change regularly and there's a decently-high turnover rate on some teams. The company is still quite small and some teams are understaffed while the company ramps up, so you may have to do a lot of grunt work.