I just had another smooth Fedora version upgrade with ZFS on Linux
When I gave in to temptation and started using ZFS on Linux, one of my big concerns was whether it would cause problems when I upgraded from one Fedora version to another. In my initial report on my experiences I wasn't able to say anything here because I hadn't done a version upgrade yet. Well, now I have; in fact, I've gone through three of them now (to Fedora 21, 22, and now 23). So I can say that for me, this was problem free. As I expected a year ago, it was basically like installing another kernel; DKMS rebuilt everything for me and it all just worked.
With that said, I think there are two important things that help me a lot here. First, Fedora keeps kernels basically in sync between their major versions. This means that a Fedora upgrade is very unlikely to turn up an incompatibility between ZFS on Linux and a new kernel (an extreme case would some change that means ZoL's kernel modules can't be built). I also stay pretty up to date with ZoL's development version, which means that I have the latest kernel compatibility fixes; as a result, I've never had problems with applying Fedora kernel updates in general.
Second, I do my Fedora upgrades via a live yum (now dnf) upgrade. I suspect that DKMS kernel module rebuilds work fine in other upgrade mechanisms, but there are at least more things that might go wrong there simply because things are happening in an environment that's at least somewhat different from the normal one. While a lot changes during a live upgrade, it's still reasonably close to a normal environment for rebuilding DKMS modules.
(Possibly this is just superstitious reassurance.)
As I mentioned back then, I do take the precaution of doing a test upgrade of a Fedora virtual machine (with ZoL installed and a pool running and so on) before I attempt the real upgrade. This can also be a reasonably good way of finding (and investigating) other upgrade surprises, although some things only become visible afterwards. Doing such a test VM upgrade doesn't take too long each time and I figure it's a good precaution to take in general (along with upgrading my work laptop first, because that's more dispensable than my office workstation).