We are talking about the Linux port, which isn't really the same functionality that's already running on Solaris/FreeBSD. I've been running ZFS on FreeBSD for a while and it works pretty ok, but even that isn't the same as the functionality on Solaris.
The FAQ even clearly states that they only support ZVOL operations. Kinda like mdadm and lvm2, but without POSIX FS compability. Anyhow, my point is that this isn't "production quality" either.
But you are right of course in that the on disk format for BTRFS hasn't stabilized yet, last time I checked, and that it is a project in its childhood.
With Oracle slashing it's Sun acquisition into profitability and the fact that the licensing issues still surround ZFS and their apparent doubling down on the btrfs effort it may not be ready for a few releases but in the longer term it definitely shows more promise especially if they end up slowing zfs work. ZFS is certainly an amazing and well written fs, but its starting to look like its future is a little uncertain.
On the other hand, this is just a port of actually working and stable FS already used in production with Solaris (and mabye FreeBSD?). Btrfs won't be ready until next few releases.