Pros
Strong engineering culture --- belief in doing it right, investing in the right tools, hiring the brightest people, and then getting out of their way. As a software engineer, you have a great deal of freedom and flexibility to work in different areas on different teams. There's no micromanaging or other BS. In the rare cases where the company asks for extra hours, there's a clear business reason. Interesting product innovation --- Arista is bringing a modern software approach to the old business of data center networking, meaning Linux, open source, open APIs, shaking up the space. Real market success for such a young company --- Arista's products are deployed at scale in the largest web-2.0 companies, financial services, telecom carriers, high-performance computing installations, etc. Lots of upside potential in the stock. Experienced management team --- industry veterans who really know the space. Cafeteria is very good (but not free, alas).
Cons
Lacks some of the hipness of "social media" or "mobile e-commerce" or "web 2.0" etc. At the end of the day, it's data center networking.