After all the hype, I expected a very difficult interview and I was right. The first two (2 phone interviews) were quite standard. I didn't sleep before my in-person interview (as in zero hours of sleep) and that made for a tough interview day.
HubSpot moves quickly throughout the process, but they also let you know about each step (spanned 3 weeks) after the weekend so you don't have the weekend to prepare. Recruiter was super friendly. After reading the reviews here I knew they'd call after with feedback, and I knew I wasn't going to get the job. I'm not sure how valid the feedback is, but it's unusual for a company of their caliber to do that. I like that they move quickly, even to reject, instead of leaving you hanging. I'm not sure how happy I would have been there anyway, but it seems like an awesome company and at least I can say I interviewed there. Also, they didn't ask "Teach me something."