I also wonder the same. I was considering using BTRFS because it's better integrated, but if ZFS is more stable/mature, I'd go with that without even thinking. Has anyone used it for some amount of time?
What I am looking at doing is getting/building Solaris compatible box for my backup needs - that is a daunting task. But if I could do that I can run one of the OSS variants of Solaris - Joyent SmartOS, Nexenta etc..
I recommend at least trying ZFS out in a VM, I guarantee you'll be impressed by the versatility.
But know that to really use it, you want decent CPU speed, as much ECC ram as you can put in there, an SSD for cache (just one - doesn't have to match the raid size I don't think - it'l help a lot) and you still need to take into account all the normal raid cautious everyone ignores like rebuild times for 2TB sata2 drives -vs- failure rates, etc.... and if you want to use dedup without verify, that's your gamble.(smarter people than me say it's safe, but it just smells wrong)