PSA: Go with Gusto (formerly Zenpayroll) instead for payroll. We signed up for both Zenefits and Zenpayroll several months ago. Zenefits was a nightmare. Zenpayroll has been an absolute delight.
They obviously don't offer identical functionality, but they're getting more similar now so this is a comparison of where they overlap.
Customer support: Both Zenefits and Zenpayroll are very responsive, but Zenefits provides significantly less helpful responses, the kind you'd expect from first-level big-corporation support. Zenpayroll is much better on this front.
Software: Zenefits is clearly not a startup driven by product or engineering. It's fine given it's free, but still has tons of rough edges and bugs everywhere (e.g. it's easy to click submit twice all over the site and create duplicate entries.) I know it's better than the incumbents, but Zenpayroll has a far better product and it really shows even in basic usage-- it's kind of amazing this level of quality now exists for payroll.
Health insurance: If you have an extremely simple situation and are a startup in CA, Zenefits is probably fine. We have about a dozen employees based in DC, SF, and NYC. Instead of telling us to go elsewhere, Zenefits spent two months and dozens of hours/email threads/phone calls making slow "progress", telling me information that ended up being inaccurate, omitting critical information, replying to one out of every three questions, saying repeatedly it would get done, and finally telling me they just couldn't do it.
The Zenefits rep could not have been worse-- if he'd been actively hostile or more clearly ignorant, that would have been better since we would have short-circuited sooner. (Yes, I did complain about the rep... they routed me right back to him on the same call.) Afterward, I called Zenpayroll and their rep was super knowledgeable and set expectations clearly. It was night and day. We also tried talking to Zenefits' partner Ubiquity for a 401k, and had a negative experience there as well.
We've now heard many, many versions of our story above. I wish Zenefits all the best since they've made a lot of startup founders' lives easier (including ours, for new hire onboarding), but boy do they have scaling issues... all I'm saying is, if you have the option of both Zenefits and Gusto for a particular service, 100% go with Gusto.