Pros
There is the chance to work on some interesting and challenging technical problems. Management is open to trying new tech and solutions, which isn't the case for more conservative start-ups. Coworkers are genuinely good people and a joy to work with. Another review said that it's a good stepping stone, which I would agree with. If you don't get too attached and can put up the right kind of boundaries, it can be a solid opportunity.
Cons
Product priorities change quickly and often with the justification that we're a start-up in a fast-paced space. It's difficult to know what you'll be doing on a long-term level. Engineers are looked at as man hours to be slotted in to poorly defined projects and are expected to meet unrealistic estimates. People are sometimes reassigned to a different project every sprint or two. At the same time, expectations are very high and simply "getting the job done" is not good enough. While there are some interesting things to work on, most of the time (I'd say 70% at least) is spent battling technical debt accrued for the sake of shipping quickly. Time to pay off technical debt is promised once a project is shipped, but since projects are late and the next burning thing is waiting, this doesn't actually happen.