I'm using on my home fileserver now, as of about 6 months ago. First time I've used it.
Impressions:
1) if anything goes wrong "how do I get to my files" is worrisomely complex. If not for how many times I've read praise of it from people who seem to know what they're doing, I'd have written it off as a great way to work really hard at eventually losing all my stuff.
2) The UI feels like using git. Not in a good way. It works but instead of writing the command to do the thing you want, you always seem to be writing three commands to do the things that (not-at-all-obviously) do the thing that you want. Nothing at all about it feels like managing disks or filesystems traditionally.
3) It doesn't like external disks very much. I'm using it with some anyway but it's not a great situation. Notably if they change designation (say, you switch USB ports) it freaks out. There are ways around this but all the examples you see on the web have you doing it the "wrong way" (at least on Linux) so if you set it up your mirroring and such with those then you're stuck with a working, but fragile setup, and the option to break all of it with unclear consequences in order to do it "right" (see 1 & 2 for why I hesitate to do this)
The data integrity assurance, checksumming, and auto-repair from a second source is great. I... kind of hate everything else about it. If I could get just those things from a more traditional filesystem that'd be really nice. I imagine it's one of those things that are fine if you live in it, but touching it every now and then on a hobbyist basis is something I'll happily abandon as soon as I can get the parts I care about more simply somewhere else.
Yeah UUID would be the way to go. That it doesn't do that anyway when you address them as /dev/sdX, like most of the examples online do, seems odd. I was pretty surprised to find it was tied to the system-ordering if you add them the user-friendly way—you've gotta do the translation to UUID for it, when adding the disk.
Since restoring a disk that had changed its system-ordered address meant re-silvering the whole damn thing (even though the data was already the same?) I was reluctant to do that a third time to fix it by changing to UUID. For now I just don't touch the disks. Eventually I'll replace the externals with internals and then I'll take care of it.
EDIT: that's exactly what I mean—examples strongly favor /dev/sdX (or similar) but in fact you want to use UUID and the zfs command line tools don't warn you, let alone default to transparently using UUID instead unless told otherwise, either of which would be a clear improvement.
Nothing in this article was new to me, but it is so far the best intro to ZFS that I've seen.