We ended up using OpenSolaris for a project specifically for ZFS, plus some related hardware support. I typically use OpenBSD or Debian for most projects requiring a BSD or Linux, so I found myself comparing it to those two systems.
We've definitely had some trouble with getting packages to work correctly, especially when using newer versions of software that were available as packages in the distribution. Most maddeningly though, it's slow. We set it up on a pretty beefy abmx machine, and at its worst it took over 15 minutes to boot.