One thing which I haven't seen mentioned is hobbyist usage, especially on surplus UltraSPARC hardware. I recently acquired a used Ultra 5 on which is now installed Solaris 11 Express. So far, my only issues have been getting a working X configuration and getting a Prism2-based WiFi card to work (PCI, supposedly supported by the pcwl(5) driver).
I'm an old Unix/Linux geek, but my last Solaris exposure was Sol9 on a Sparc 20. It has, so far, been interesting to learn about some of the newer innovations such as ZFS and the new service handling and administration.