Pros
Fantastic mission - You can work on anything as a software developer these days, it's great to work on something that makes a difference. Great size - Small enough that there's very little red tape, and individual can have a significant impact. Big enough that the pay and benefits are good, you can take time off without the business shutting down. Great people - Haven't encountered any jerks yet (been here 6 months so far). Co-workers are passionate about the mission. Strong momentum - Sales team is doing a good job of closing deals and the company is investing in new products. Customers seem to really love the products and the brand, which makes it easier to bring additional products to market.
Cons
Growing pains - Company is growing fast! This creates a lot of opportunities for everyone, but it also means systems/tools/etc can lag a bit as we scale up and level up.