Solaris 10 is free for both x86 and sparc versions. You don't get any support, and have limited access to patches, but you're allowed to use it (even in a commercial server situation). Solaris 9 is less free; I think it's uniprocessor only, non-commercial.
As an X server my Ultra 2 (dual 300MHz 4MB cache, 512MB RAM, 73GB HDD) feels about as responsive as my P3 700. Load times are quick thanks to the HDD and it multitasks very smoothly. It really shines under heavy MySQL loads though: it just keeps on going no matter what sort of load you pile onto it. Thow a similar load at a single (IDE-based, Linux) XP1700 and it does the typical overload collapse. It's probably a whole lot of little things adding up (SCSI, nice bus architecture, Solaris, SMP) but as a whole it's certainly a tank of a machine.
On a price/performance scale, is it worth it? Hell no, for the same money I almost certainly could have got much better performance using a modern CPU and an off-EBay SCSI system. But it's also wonderfully built, much better than any other recent boxes I've messed around with. I've had a GDM 20E20 monitor sitting on top of this box. According to Sub it weighs about 30KG, and my arms would tend to agree with this number. The box takes it with no trouble. Sit a similar monitor on top of a, say, Dell desktop case and you'll end up with a pancake. Everything inside the case is also screwless. Flip a (metal) lever and the HDD/CPU/RAM/fan smoothly slides out. It's wonderful to work with after cheese-grating my fingers inside my main computer. Only problem is, there's rarely any reason to open it up.
OK, I'll stop reminiscing. Usual thread activity may resume