Well, we (and by this I mean myself and my fantastic team) have been running it since 2015 as the main filesystem for a double-digit number of KVM hosts running

Well, we (and by this I mean myself and my fantastic team) have been running it since 2015 as the main filesystem for a double-digit number of KVM hosts running a triple-digit number of virtual machines executing an interesting mix of workloads, ranging from lightweight (file servers for light sharing, web application hosts) to heavy I/O bound ones (databases, build farms) with fantastic results so far. All this on Debian Stable. The setup process was a bit painful given some interesting delays when using some HW storage controllers that caused udev to not make some HDD devices available under /dev before the ZFS scripts kicked in and we have been bitten a couple times by changes (or bugs) in the boot scripts, however the gains provided by ZFS in terms of data integrity, backup, and virtual machine provisioning workflow were definitely worth it.