ZFS has been around a bit longer than Btrfs, which the ZFS proponents claim has allowed for it to become more stable. Watching the commit logs of Illumos (the open-source derivative of Solaris) most commits seem to be related to adding features or reducing IO latency variance. Problems with lost data are few and far between.
As an appeal to authority, a number of companies currently trust[1] their data to ZFS, Joyent probably being the most well know of them. I store my personal data[2] on ZFS, though my needs are modest.
