Pros
I think the thing I love most about HubSpot is that it's at a place where we're established enough to have the resources and the wisdom of a company that's figured out what we're doing, but still small enough that a single person can make an enormous impact. For a designer like me, that's huge. At a larger company, like Facebook or Google, I'd never have as much autonomy to make the design decisions that I currently do as the sole designer for my piece of the HubSpot software. That's given me an incredible opportunity to grow my craft and has made me so invested in the part of the product that I work on.
Besides that, the people I work with, especially my immediate team, are fantastic people - they genuinely care about their coworkers, the company, and the product we're building, they're smart, they're helpful, and they have good senses of humor. I couldn't imagine a better group of people to work next to every day. In addition, I've never had a problem with work life balance at HubSpot - I set boundaries for myself before I started working full time (like not working on nights or weekends except in extreme circumstances, not responding to emails or chats after hours, etc). Of course it's easier for my role, since I don't have to be on call like engineers do in case something breaks, but I've found that my coworkers are very respectful of my work hours.
In general, HubSpot is a fantastic place to work, and I'm incredibly happy at my job.
Cons
Things change as the company gets bigger, and as it gets bigger it gets a little harder to make sure the right information gets around to the right people. There are a few growing pains like that, but they certainly pale in comparison to the pros.