Peter, I've not had hands on experience with Btrfs at this point. Though I'd trust it on a NAS a bit more since NetGear already switched the ReadyNAS line over back in 2013. That at least added a larger group of "beta" users and likely led to a lot of improvements and some stability time.
My thoughts too Tom, to some extent. I would be more inclined to use btrfs given the weight and reputation of some big name business NAS vendors such as Synology and NetGear behind it.
Currently, my thoughts are to wait until DSM6 comes out of beta, then upgrade my DS710+ to a DS716+.
As mentioned previously, my main motivation is the snapshots which I am hoping will be a more robust and granular method of 'versioning' than Time Backup. I believe this part of btrfs is quite well tested and stable now.