I'm not going to pretend to be unbiased here, nor will I address the behavior of the Zenefits CEO. (I will note this was an A2A, however, and it's likely I wouldn't have answered otherwise).

Come work for Uber. We need great engineers, senior, junior and everything in between. Why? Because we are trying to solve hard problems everyday and there are more great ideas in our offices than people who can build the technology and write the code to implement them.

Oh, wait, that's not a great sell. It explains why we want you and very vaguely states what you might be working on, so let me dive in here with some more detail.

• We're a complex, global business but still a fast-growing startup. Uber is in 57 countries and more than 300 cities. It helps make more than 1 million rides happen every single day. But we think we're at the beginning of a transportation revolution. The first inning of a baseball game (maybe the second). The first quarter of a basketball game. The opening moves of a chess game.
  
• We're 2,000 plus people around the world and, to me, some of the most extraordinary. The talent level is super high. It's not easy to get an offer at Uber. If you did, it means you impressed some serious people who had other candidates to choose from. They chose you to work among them. You'll probably learn a lot even if you end up being one of the superstars in the Uber galaxy. The best thing about working for Uber is being among such an extraordinary group of individuals.
  
• The second best thing about working for Uber is the mission. In no way do I wish to suggest there aren't other startups doing great things -- there are plenty. It's also possible some startup doing something that today seems trivial ("Let's replace the freshman picture book") will tomorrow be just about the most important internet property in the universe (Facebook now connects ~1.5 billion people). That said, Uber is committed to offering transportation as reliable as running water anytime, anywhere -- at the lowest price possible. We want to help the world move away from single individuals in owned cars to shared vehicles that often carry 2-4 people in carpooling at massive scale. This is the planet's best chance to reduce congestion, emissions, the need for parking, et al. At Uber, you can help remake cities and change the world.
  
• The tech organization has some extraordinary people in leadership. You might not have visibility immediately with Thuan Pham or Pedram Keyani or the other amazing people helping run things in engineering, but you'll benefit from how they do things from day one. And the chance to grow and contribute in a great engineering organization is going to benefit you in your career, not just at Uber but wherever you go next -- though we'd certainly hope you'll stay a while. Terrific people aren't a dime a dozen in the Bay Area tech world. We're blessed to have a bunch of them.
  

I don't know what your offer looks like nor do I want to trivialize getting more money elsewhere. That said, I doubt very much you'll ever regret coming to work at Uber. Hopefully, we'll see you there soon.

(Though I work there, the above isn't an official statement of Uber.)