Pros
If you want to work for a pre-IPO software company on the way up that creates interesting technology then this is a great place to be. Engineering has a ton of initiatives in progress, so there are always challenging projects to work on and new technologies to learn. Most of the company develops in Java with web frontends built using AngularJS, but newer projects are written from the ground up using Node.js. Backend platforms heavily utilize Amazon Web Services, MySQL, PostgreSQL, MongoDB, ElasticSearch, Hadoop, etc. There is a very high bar to entry, so all the people who work here are really smart and interesting with good attitudes. As an engineer I mostly work with other engineers along with some people in professional services, product management, documentation, HR, and marketing/sales. In general the atmosphere is very collaborative and friendly with little political B.S. The company is growing rapidly, so you won't be employee number 10, but you also won't just be another face in a crowd. Most everyone knows each other by name and management does a decent job keeping people in the loop of what's going on at the macro-level. You'll find plenty of people enjoying a beer together in the common area Friday afternoons. Salaries are pretty good for engineers, but obviously won't quite match what you can get at an established large company. That's where the hope for an equity payoff comes in.
Cons
Nothing serious to complain about. Occasionally you'll find some dysfunction, which is inevitable whenever people from differing backgrounds work closely together, but it's always very minor from what I've seen. The sales and marketing teams can be a little more high-strung, but that's just the nature of those types of jobs and is common to every company I've worked at. Engineering and product managers are very busy, so you won't get a ton of hand's on direction, but that shouldn't be a problem if you're a self-starter. The products are not the latest sexy consumer fad, but the underlying technology is very interesting to work with and the business applications are huge.