What do you like best?
Keen.io includes all the tools you need to get a grip on behavioral analytics for web or mobile, from a developer interface. There's no black box. You know exactly how your metrics are being calculated, and what's happening.
What do you dislike?
Product probably isn't suitable for those who are trying to "figure out" analytics in their organization. Other products charge more, but offer much more support as far as libraries & guides go. However, Keen.io has an awesome slack channel where you can ask anyone literally anything, so a tech savvy beginner has a shot.
Recommendations to others considering the product
If you've got an experienced data team and dev support, at minimum you should evaluate Keen.io before spending >$1,000/mo on Mixpanel/Heap/etc.
What business problems are you solving with the product? What benefits have you realized?
Keen.io was perfect for solving analytics problems before I had a budget for more expensive analytics tools, or to build a data warehouse. It scaled really well and met all of our KPI / behavioral analytics needs.