Pros
The biggest reason is the people. Formlabs does a great job of hiring employees who are not only passionate and competent, but also just really great people. I enjoy spending time with my peers at work, and consider them to be friends. I also have a chance to make significant impacts on Formlabs products and feel like my work is impactful to the company as a whole. Other great pros: Flexible hours and time off, catered lunches three days a week, competitive pay and benefits, oh and did I mention the people are great? Also, there's a bike room for people who ride to work to keep their bikes!
Cons
Flat structure does mean it can be hard to get a tangible sense of career development. Work-life balance can be achieved, but it has to be a deliberate choice - there is always more (usually interesting) work to be done, and with no specific scheduled hours saying when to go home, it's easy to work late more often than intended.