True, I did not mean to say usable, I meant to say that "officially supported", but I will give it a go with a custom built kernel sooner or later.

For how long have you been using it? Any issues? Any favorite feature(s) in particular?

I've been using it about a year or so. Like I said, the 'killer feature' for me is tiering. I'm not using it on my main computer, but a spare parts computer with whatever storage I have laying around thrown into it, so being able to combine a 3TB HDD with a 1/2 TB SSD. All the storage stays usable, and I get better performance.

Not perfect though. Performance at times doesn't seem better than just SSD, though I didn't really test that and I'm not sure if it's just hitting the HDD at those times. And there was a kernel version that didn't boot, though I'm staying on the bleeding edge with Arch Linux.