We use Zen databases internally in a wide variety of solutions, from periodic data logging (from our PV solar installation) to web search capabilities (of our accounting data), and even data analysis where we can import large data sets to the database, and then slice&dice it with other tools to glean insights about our data.

The speed and flexibility is far above everything else, allowing a wide variety of access methods to my data, leveraging low-level API calls and high-level SQL/scripting access to the same data, at the same time. The minimal need for management on a daily basis is also a big plus, as you can just set it up and let it run.

Data replication to another live server is still a bit troublesome, but the availability of several backup and DR solutions, along with the ability to spin up secondary servers at a moment's notice, is enough to overcome much of this issue.