OpenSolaris is 1 year old today! Congratulations, and thanks to everyone that made it possible!
I've used SunOS/Solaris for ~20 years now, for all of the old school reasons: Solaris has long set the gold standard for things like reliability, scalability, observability, debuggability backward compatibility, and standards compliance. Over those years, I've written code that had to run on most versions of Unix as well as VMS, Windows, and the Macintosh. Sun was always the place where I did initial work, and any possible debugging as well. The reason is that SunOS has always had superior tools for observing and diagnosing what your code is doing. However, the user visible functionality of the OS (as with all Unix OS's) had not changed much in quite some time. Solaris still had the above advantages, but the various Unix variants were beginning to look and feel increasingly similar.
In contrast, the current Solaris is a real jump forward. While other operating systems have been working away at duplicating things that Solaris has had forever, Solaris itself has moved forward with next generation features that no one else will have for quite awhile (Dtrace, ZFS, Zones, FMA, SMF, etc). Others have some subset of these abilities, but no one has the complete package, and no one else has it in such a simple and fully integrated fashion. Once you experience these things, you'll find running systems without them to be limiting (evoking a quieter, simpler era). At the same time, Solaris has moved aggressively to the 64-bit X86 PC platform, making it possible for many people to run it without first having to buy new hardware, and has moved to adopt a modern desktop and make other needed improvements (many of which involve open source code from other projects).
The fact that Solaris has joined the community of open source Unix operating systems makes the above even more compelling. The energy level surrounding Solaris today is high. The quality of Solaris as a desktop OS is rapidly improving, thanks to its adoption of other open source software. At the same time, the OpenSolaris code is there for others to examine, modify, and even to port to other systems This would be a great thing for Unix in general, and is something that we would love to see happen (and it is: See Dtrace and ZFS). And, Solaris still leads the pack with those boring old school virtues.
So on this 1st anniversary of OpenSolaris: If you are a Unix fan who is not familiar with the modern Solaris OS, you should give it a try. You will find it interesting and eye opening. Rest easy: It has a real open source license, and what has been given cannot be taken away. You probably already have a PC lying around that you can use. And the price (free) is exceedingly reasonable.