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.
Last time I experimented with OpenSolaris, it seemed as though it was very useful for Java development, but not a whole lot else. It might have changed since then though.
What does it do better than Solaris 10, which seems about the equivalent of AIX or HPUX in terms of installation, package deployment, and toolset maintenance (as little as possible).
I fear most for OpenSolaris. It's harder to fork than VirtualBox and far bigger.