I've installed OpenSolaris 2008 11 (now declared as final) twice on VirtualBox (Mac and PC hosts) and once on the bare metal (a Toshiba laptop). Installs worked very smoothly (with the exception of a small Grub glitch in the later case), the UI is polished, the Update tool is more reactive (we use a very similar version in GlassFish v3 Prelude), as explained previously GlassFish v2 is on the repository and v3 will be there in the future.
ZFS is the primary filesystem and its first nice feature is that you don't notice it. The second is the Time Slider capabilities which you can see for yourself here in Erwann's short screencast. Power management seems to be greatly improved (too early to confirm). On the down side, there's still some time to boot (clearly not an issue with VirtualBox) or even shutdown. Roman's 12-minute screencast is another nice intro.
If you're an OS freak regularly trying out releases of Fedora, SuSE, Ubuntu, etc., you should try OpenSolaris.