Pros
Good pay, nice benefits, nice, beautiful office, interesting problems to work on, successful company. If you manage to keep your head down you can probably coast pretty easy here.
Cons
Code quality is quite low. Many, if not most systems at Datadog were built by one-man "teams" of engineers, often as hack-day side-projects, rapidly launched to production well before they were ready. Applications are almost never built in a "normal" way and there are whole applications without a single test. As a consequence systems break frequently and no one even knows how they're supposed to work. As a competent developer, you'll probably notice this in your first two weeks, but you'll also soon discover that there are many sacred cows at Datadog, and despite their obvious deficiencies, the design of these systems is largely non-negotiable. Most of your work will consist of a constant barrage of surprisingly crude devops changes just to keep the thing running. For many teams, most work is unplanned and very stressful because it's done in response to outages. (we have 3 or 4 outages every day, so half your day will be spent working on "urgent" problems) Backend engineers also participate in the on-call rotation, which ends up being once every 4-6 weeks and lasts for a week at a time. You will be responsible for 90% of the systems, even though you know nothing about them, and because everything is mostly broken, you will be paged every hour. Also you will get to experience the joys of working with an international team, where because almost every change at Datadog results in an outage, the developers in France will consistently wake you up at 4am every morning. The company has grown rapidly, but management has not kept pace. Team leads have no experience in leadership, do no planning and make irrational, heavy-handed decisions. Concerns are not taken seriously and you'll be treated like a junior engineer regardless of your prior experience.