Process consisted of one or two phone discussions and an on-site 4-hour interview with an SE, the head of customer success, then each of the co-founders (separately).
The phone interviews with HR and customer success were straightforward discovery. I think HR covered salary early on.
The on-site was a standard tech-company interview process to vet technical capabilities, problem-solving prowess, and personality.
1. The SE asked me to write some basic code to do something like reverse a string, as well as present any concept of my choice. This piece went pretty well and the guy was super personable.
2. The customer success manager was pretentious, yawned the whole time, and had bizarrely unwelcoming body language. He asked me about my experience and finished up with a brain teaser. The interview itself was OK, but he blew me off after the interview process (sent me an email afterward saying he would chat w/ me about it, then never responded) so my overall impression of this guy was negative.
3. The CEO was less than pleasant to talk to. He's clearly intelligent and has the drive necessary to build and run a company, but I couldn't imagine working for him based on his smug attitude. This was my first professional interview in a long time and I hadn't rehearsed / contemplated some of the questions he asked, but my inexperience was moot when the conversation devolved into a "how many hours will I work" discussion.
4. Last exercise was a customer support role play with the other co-founder. Despite me making an assumption at some point in the role play (which led me down an incorrect path) it was a pretty fun exercise. At this point I realized they were looking more for a high-end support rep than an actual sales engineer.
Mixpanel looks like a good product and an interesting opportunity if equity is attainable, but I'm glad I ultimately didn't get an offer. I ended up getting and accepting an offer for way more money right after this interview (not in the Bay Area) that seems to have a much better work-life balance and an equally impressive group of people.