Once this becomes available I will have one fewer item on my list of features missing from ZFS.
Currently to limit users' consumption the workaround documented here is to provide each user with a dedicated directory on which another dataset is mounted and a quota is set. This implies that the user can only create or write to files in that specific directory. To track and limit a user's total usage across an entire ZFS pool requires User quotas - ditto for consumption by group.
According to this post by Matty the feature is implemented in a way which enforces the rule "tardily", that is it is a little "late", and also mentions that translated SIDs (eg when the directory is shared via SMB) are supported.
The PSARC/2009/204/ document here provides details of how the quotas is implemented. Two new zfs subcommands, namely zfs userspace and zfs groupspace reports the consumption, and control is by means of a set of new properties on ZFS file system datasets.
This amounts to good news all around. Maybe I should start tracking bug IDs for all of the items on my feature wish-list!