We are still talking just about backup/storage. ZFS has a lot of features and it's used for about 4 years or so in production. Hammer FS don't have so much features and is "stable" for about year. btrfs is for those who want to experiment. Some cons - OpenSolaris has terrible dev process, but you must use dev if you want update and security updates, but there is a lot of bugs in those versions. Solaris is not free anymore including security updates after change in rules before one week. Support for ZFS in FreeBSD is marked as experimental, but it depends. So Hammer FS looks like most promising regarding feature on other BSD systems (just my personal tip)
ZFS in FreeBSD isn't considered "experimental" since last september 2009: http://svn.freebsd.org/viewvc/base?view=revision&revision=197221 So, it's considered production-ready at present.
