I haven't tried the OSX ZFS stack myself, but this is a quote from a friend who tried it back in June:

"Currently shuffling all the data off my Zevo zfs formatted drive on my MBP so I can reformat it back with bad old HFS+

I can cause system crashes that are directly caused by Zevo not handling particularly intensive bouts of disk activity on large numbers of tiny files. Which pretty much describes how Aperture hits its database. Which is pretty mission critical for me - could do without any instability, least of all something that hits my photo databases."