Solaris is more Unix than Linux, though like Linux, it is techincally a Unix-like OS. Sun has had 64-bit CPU's and 64-bit Solaris for quite awhile, they just don't really measure up to Athlons or P4's.
Solaris is very stable, and there's a 32-bit version that can run on x86 systems. I don't really like it as much as Linux. I've used Solaris 8. It's desktop is useable, but not as nice as KDE. But I don't have enough experience with it to really say much else about it.
I wound up trying to install the Sparc port of Gentoo onto one of the Sun boxes I managed- so users could have KDE and OpenOffice and things like that. It didn't work out so well. I got the base system on fine, but I couldn't get KDE to complie.