Pros
Mainspring is an exciting, quickly growing startup with a truly innovative product. It's full of smart and passionate people who care about solving energy issues through cutting-edge technology. Everyone is generally good to work with, and willing to work hard to support each other. Since it's still a small company, the culture is relatively non-bureaucratic and transparent; everyone has the chance to learn about the high-level company goals, schedules, trajectory, and challenges. I've also had highly positive experiences with management and general company leadership. I've been truly grateful to work at Mainspring, where I'm engaged and challenged in new ways every day. I'd definitely recommend to anyone who cares about alternative energy tech and is willing to work hard to help develop a product with immense potential.
Cons
There have been no major cons for me working at Mainspring. Certainly everyone is expected to work hard and be a team player, and put in extra hours when necessary. The workload can be challenging, as there's always a lot to do. That said, I've been able to achieve a very reasonable "work-life balance."