Pros
The Allen Institute for AI (AI2) really is a special place to work. It offers a unique opportunity to work with some of the world's best in natural language processing, computer vision, and artificial intelligence. AI2 has a strong culture of learning and no matter what role you have here, you'll find yourself growing everyday. I held roles in both engineering and research and always found this to be true. For researchers: AI2 provides an unparalleled environment for magnifying the impact of your work. Creating a dataset? Launch a leaderboard! Building a new model? Make a demo! The software, infrastructure, and colleagues available to you at AI2 supports, encourages, and makes it incredibly easy to go the extra mile with your work. For engineers: AI2 offers the opportunity to tackle interesting problems at a scale where every engineer counts. You'll directly see the impact of the work you do: building products, running services, supporting research. In addition, you'll get regular exposure to the cutting edge of AI, as well as amazing engineering colleagues. Since the institute is a nonprofit driven by its mission (AI for the common good), the organization's goals align well with what I've often found motivates myself and other passionate engineers: wanting to build things that help people.
Cons
It's not quite a con, but important to be aware of: AI2 is a highly collaborative organization which plans its work around central themes and goals. AI2 takes a mission-driven approach to setting research directions, organizing its efforts around the different projects (Aristo, Semantic Scholar, Mosaic, etc). To be successful here, you must be excited to contribute to the long term goals represented by these projects. You should be able to shape your work around these big picture visions.