Pros
It's a great place to learn a lot about marketing and entrepreneurship, and a fun office environment. There's a lot of autonomy, and as long as you hit your metrics you'll have a long leash, and flexibility. One of the only companies I've come across that truly encourages you to move around between depts. The biggest asset are the people work there. Everyone is super smart, and they're quick to fire employees not working out. It leads to a really top notch organization.
Cons
They hired great start up minds to grow the business, but aren't a start up anymore. So those people are getting bored and leaving, or being forced out. HubSpot has a really noble mission, they're trying to transform the way a modern business operates (with progressive views on vacation, flexible hrs, transparency, etc). However, the mission is different than the reality. They're an 800 person company, and internally they function like one. Lots of red-tape, politics, lack of cooperation between depts, and comp cut-backs were happening when I left.