Pros
What makes Arista a great company to work for is the culture. A culture of 'doing the right thing', it sounds simple enough, but it is immensely powerful, and everyone follows it. I've seen companies sacrificing on quality for a release deadline. That doesn't happen here, when it comes to software quality it trumps all sales or marketing imposed deadlines. Arista has the best tools for making a developers life easier. Whether it is source code management, or for running unit tests. A lot of people have put in lots of effort to build infrastructure to make the developers life easy. I've seen new products being released in amazing speed, like 6 to 9 months, which bigger company like cisco would have taken 18 to 24 months to do. Central to EOS is a meta language that makes writing code easier. Eliminates writing lots of boiler plate code (if you've ever read Effective C++ book, you'll understand the difficulty in writing good C++ code, and this takes care of a lot of it). Another part of Arista culture is helping fellow engineers and seeking help. Performance review process is designed to encourage engineers to be helpful to others. In previous companies, performance reviews and ratings were pretty much at the mercy of your reporting manager. At Arista there is a democratization of reviews (peer-reviews) which is awesome! Very hands-on leadership team, most of the infrastructure and tooling were written by engineers who are now in management positions. You won't find a PHB in Arista, and any inputs from management is usually constructive and strategic rather than clueless or political. Lots of freedom to choose projects to work on. Lots of freedom to switch teams/managers. Perfect work life balance, there was a crunch time for one quarter when everyone was putting in 50hrs a week, but things have returned to normalcy now.
Cons
Engineering organization is almost flat (few hierarchies), there are no titles for software engineers. It feels like there is no ladder to climb on the technical path. Reviews are conducted anonymously only the manager knows the feedback if you're doing good or awesome. Still, it is difficult to guess where you stand in the ratings because the feedback is subjective and no metric is used in the reviews. No definite IPO date. 3 CFOs have changed recently. Compensation not on par with competition.