Inspired by reading Jeff Bonwick's Blog I decided to give it a go on my development gates. A lot of files are shared between clones of the gate and even between builds, so hopefully I should get a saving in the number of blocks used in my project file system.
Being cautious I am using an alternate boot environment created using beadm, and backing up my code using hg backup (a useful mercurial extension included in the ON build tools)
I'm impressed. As it works on a block level, rather than a file level, so the saving isn't directly proportional the number of duplicate files. But you still get a significant saving, albeit at the expense of using more CPU. It needs to do a sha256 checksum comparison of the blocks to ensure they're really identical.
Enabling it is simply a case of
$ pfexec zfs set dedup=on <pool-name>
Though obviously you can do so much more. Jeff's blog (and the comments) are a goldmine of information about the subject.