I use Solaris Express on my desktop. SX is released once every month, since it's a preview (beta) release of Solaris 11, which will come out next year. These builds have better driver support and new versions of software compared to Solaris 10. And SX is rock solid, not like Windows/Linux beta releases.
A few things to remember: since Solaris is primarily a server OS, don't expect the same features, driver support, software or repositories as any Linux distribution or Windows. There are two software repositories for Solaris x86 - Blastwave and Sunfreeware. Solaris Express has GNOME 2.14 (JDS) and traditional CDE as desktop environments, and Xorg 6.9. Work on GNOME 2.16 and Xorg 7.x is in progress. WiFi and bluetooth are not supported yet, though a few WiFi drivers (like Intel 2200BG) are available for download separately. DRI has been merged in snv_52, but it's still not available for download. OpenGL is only supported in case of nVIDIA cards, for which the gfx company provides the drivers itself.
In all, Solaris is great for servers and/or learning UNIX. It's the only fully POSIX and SUSv3 compliant system - the real UNIX. I think, IBM's AIX also has the same certification though. Solaris has excellent backwards compatibility - you can still run binaries from early 90s. It has an excellend driver model, and the code quality is vastly suprerior to any other open UNIX(-like) system. Should you ever need a C/C++/Fortran95 compiler, Sun Studio has been made available free for all purposes - including commercial. Solaris also has extensive Java support.
Feel free to ask questions.