Or even Solaris proper. Solaris 10 changed the game from the ground up, much to the point where it's Unix on roids. Run levels have been replaced with milestones, init.d has been replaced with SMF and the contract file system. Dtrace makes life worth living. Look, vmstat is great; but with Dtrace you can recreate vmstat/iostat/mpstat from the ground up! Get the picture of what this tool can do for you?!?! Containers/Zones for virtual hosts. OpenCluster for building and working with an HA cluster. Crossbow, for building whole networks inside your machine.
I think my only complaint about OpenSolaris is packages. After 8 years of Debian apt calls I find *Solaris to be a little too retro-RedHat (before YUM) for dependencies and new software.