Pros
Truly the definition a booming startup. This is what I imagine companies like Google and Facebook to have been like while they were starting. Benefits are great, even for interns, and all the same benefits that apply to employees apply to interns as well (aside from the usual medical insurance and stuff like that) such as free lunch/dinners, work from home, company events, etc. Work is super interesting no matter what team you're on and since the company is still relatively small, you'll be given a lot of responsibility which is great for professional development! Engineers are cream of the crop and come from big-names such as Google, Microsoft, Apple, etc.
Cons
There are none! The only thing I can say is that sometimes if you need on a project help it may be hard to get since everyone is so busy, but once they do, they're always willing to give assistance