I think the whole thing stinks from both sides. Zenefits' blog post does seem to walk the narrow line between informing their customers and defaming ADP. Meanwhile, ADP files a defamation lawsuit then turns right around and releases their competing platform, which could damage their case (to my non-lawyer mind, so I could be completely off base there).

I can see merit in both parties' cases against the the other, but I can see a ton of shenanigans on both their parts as well.