There is Zfsonlinux and ZFS via fuse, and they have different advantages and disadvantages.
FUSE (filesystem in userspace) is essentially a cute way of getting ZFS without modifying the kernel to do it. Now, given that there are licensing issues (not the same license), there is an advantage for distros in going this way, rather than 'hacking' the kernel to do it.
On the other hand, Zfsonlinux is faster, sometimes significantly, sometimes not.
So, you have to decide what is important to you.