Pros
The company is small so you'll have the opportunity to learn a lot. If management believes in you, they'll spend the time to mentor you. Many that have worked here as entry or mid-level positions have gone on to lead teams at larger companies because his/her experience exposed them to a lot of different challenges. The CEO isn't perfect and he isn't a great manager, but in my time at Merus it was apparent that he always wanted to provide the best product possible and cared about his employees. There's lots of opportunity to have face time with leadership, lots of happy hours, 3 weeks PTO, pretty great health care considering they're not required to provide it, office snacks, convenient location to BART and cool places to grab lunch at.
Cons
Other reviews have merit - it is a tough place to work. There aren't any processes in place since it's a really laid back start-up mentality, there's no HR or People Team, management doesn't really manage and if you're not a self-starter you'll have a really hard time at Merus because management wants you to research and find the answer before approaching them. Since the company is relatively small, there's also more responsibility given to people that aren't ready for it. So yeah, you'll learn a lot, but you also may be on the hook to stay later one day or defend a choice you made that affected a client. Because of this a lot of entry positions experience attrition.