There are a few commercial NASes using BTRFS. And btrfs can do quite a few things that other filesystems and mdadm cannot like snapshots (you could use LVM), bit rot detection, compression, etc. btrfs also makes changing raid level much easier than mdadm. It is much more native to Linux than zfs as well.
Thanks for the information. Last time I checked (a while ago, when deciding what solution would be best for my 10 drive RAID6) brtfs wasn't considered mature enough for production RAIDs (especially levels 5 and 6), but I guess it is now if it's being used in hardware NAS'es.