Pros
Feels how a startup should be. No internal bureaucracy, you're close to the founders and decision making process, if you want the product to be a success you can make it one. Generally good vibes from the other employees, who for the most part believe in the vision and product. Much more interesting technically than I thought it would be.
Cons
Office culture can be a bit lacking, sometimes. Startup has been around for a while so occasionally feels like it's lacking momentum/excitement. Of course it's a small company so perhaps harder work for less money than other places.