Here is another scenario for ZFS that will benefit the average user. Creation of filesystems is trivial in ZFS, with low overhead and the ability to share one storage pool amongst many filesystems. The classic example is to create a new filesystem for every user, but that doesn't help much on a single-user Mac.
But consider this: the average Mac might contain five main kinds of data: movies, pictures, music, documents and preferences. Each one of these types has different properties. Movies, pictures and music don't compress well (they are already compressed), but documents and preferences may compress well. ZFS supports on the fly compression. Wouldn't it be good to use automatic compression on the documents and preferences, but avoid wasting the effort on the other types? Create a different filesystem for each type, and you can enable or disable compression wherever it is appropriate.
But there is more. Music files tend not to change... I'd venture the main changes to the music file system are file creations. This makes snapshots unnecessary. Also, while preferences do change, they tend to change in trivial ways, so why bother taking snapshots of your preferences? If you want to preserve the states of files, you can take a snapshot at any time. With separate filesystems for each data type, you can snapshot only those things that change, only when you want to preserve them. There might not ben that much overhead even if you included your preferences and music in every snapshot on a single filesystem, because the files are either small or stagnant. However, splitting up your filesystems keeps organization of these snapshots easier.
There are other benefits, like per-filesystem quotas. Maybe you are a developer, and you are working on a program that generates a lot of output. You don't want this output filling the whole disk. Set a quota on the filesystem that will store the output, to avoid this problem.
In my case, I used to have three partitions on my Mac: one main partition, for the system; one partition for the /Users hierarchy, and one partition for /opt, which is used by the MacPorts system. This kept everything separate and well-organized, and allowed me to use case-sensitivity on my /Users partition without doing the same with the system files. This was nice, but it had a problem: I had to choose fixed sizes for each filesystem. Thus, while I had 45 GB of 55 GB free in my /Users partition, and 7 GB of 10 GB free in the /opt partition, I was down to the last few GB in the system partition, which was giving me headaches. I had to reformat and reinstall, combining /opt and the system partition, while shrinking my home directory a bit, to give more space to the system partition. ZFS avoids this issue, allowing me to create arbitrary filesystems for organizational peace of mind, without actually forcing me to subdivide the available space.
