ZFS has a lot of cool features. Granted. But the question is whether or not you need those features, and are they worth the cost that they incur. Especially in the days of VM's, memory is often at a huge premium --- consider how much you have to pay for extra memory. This is not surprising, because if you try to pack a couple of dozen VM's on a single server, memory very often ends up being the critically short resource (servers have only so many DIMM slots, and high density memory modules are expensive).

ZFS might be great if you're running a file server where you can pack many gigabytes of memory into a server and not use it for anything else. But if you are trying to run a cloud or VM server, where you are trying to pack lots of jobs on a single machine, the cost/benefit ratio of a copy-on-write file system (whether it is btrfs or ZFS) may not be worth it.