Pros
At Symform, we work with lots of technologies (frameworks/languages/etc.), we aren't married to just one way of doing things. I got a lot of great experience with new things after leaving the silo world of Microsoft. Employees are empowered to decide how best to build new features. Deployment to the production environment happens fairly often, so you get quick impact of the code you write. Also, the Symform product idea itself is pretty cool. A Peer-to-Peer storage network, and all the "interesting" engineering challenges that come along with implementing and debugging a distributed system, is a lot of fun that keeps me engrossed in my work. Symform also has great people. Very intelligent colleagues that are a pleasure to be around, learn from, and collaborate with. And who can forget (delicious) catered lunch every day? Mmmmm.
Cons
A start up atmosphere isn't for everyone. It can be a bit hectic sometimes and there is a lot of unknowns to deal with while building something no one else has done before. People that like working in that environment can thrive here though.