For anyone considering George's suggestion, I would like to point out a few things about Mac OS X first (after that, if you still want to use UFS, don't blame Apple for system performance and problems).
Mac OS X is not like any of the other operating systems that came before it (Mac OS X Server 1.x, Rhapsody, OPENSTEP and NEXTSTEP). All the parts of those operating systems were designed to work on UFS, their applications were designed (in what is now Cocoa) to work with UFS, but this is not the case for Mac OS X. Mac OS X has three different application environments (Carbon, Cocoa, and Classic), and two of them (Carbon and Classic) are designed for HFS+, not UFS. Even if you no longer use Classic apps, and enjoy Cocoa apps more than Carbon, the Finder is still a Carbon app. The Finder of Mac OS X is much like the Finder in the old Mac OS, it relies on information stored in hidden files and resource forks to move from directory to directory. If this was just another version of the Workspace Manager, we wouldn't be able to have aliases (something special to the Mac OS) and custom icons that can just be assigned with cut and paste (yes I have tons of custom icons in my pre-Mac OS X systems, but they are all tiff files that I had to physically put into place).
George is right about the comparison with Windows 2000 and NTFS/FAT32, only he got the file systems for the Mac OS backwards. Installing Mac OS X on UFS is worse than installing Windows 2000 on FAT32 because Mac OS X is designed to take advantage of HFS+. In all the Unix hype, we seem to be forgetting that HFS+ is where many of the features that we know and love are rooted.
This is why Apple does not install Mac OS X on any UFS formatted system.
As for installing on partitions, for most this is simply a judgment call. For those of you with Beige G3s or Wallstreet G3 PowerBooks, installing on a partition that is the first one on the drive and no larger than 8 GB is still something of a requirement. There is still the problem of allocation blocks (which get bigger as drives get bigger), HFS+ is good at dealing with larger drives, but even better if the drives are partitioned to more manageable sizes (any where from 5 to 20 GB).
The only time Apple has suggested using UFS that I am aware of is when using Mac OS X as a web server (Apache and the internet work better with files that are not tied to resource forks).