HAMMER is considered production-stable now. It was usable a year ago and what few bugs existed were found and fixed since then. Performance is much improved though we are still not happy with small file lookups, and fsyncs are still quite costly, but there isn't much we can do about it without some major on-media changes and those will probably not be made until after the cluster work ramps up. In anycase, HAMMER has been the default filesystem for a while. There is really no other choice for DragonFly (just as ZFS is the only real choice for FreeBSD) when it comes to dealing with today's huge drives. UFS (and UFS2) don't cut it.

-Matt