We've had a very positive experience with btrfs, using it on a redundant system for a few years. It's a lot more flexible than zfs in some respects. For instance, all the snapshots are modifiable and can themselves be modified. You can build up a graph of snapshots, which is not possible in zfs which enforces a strict hierarchy. We ran btrfs on top of mdadm, to get raid and integrity checking.
btrfs can do things like copy particular files using copy on write, which is really cool. btrfs also supports offline deduplication, which isn't supported in zfs. This is very useful if you want to do the deduplication when the system is not otherwise being used and avoids the overheads of keeping hashes in memory all the time.
I think that 10 years behind is exaggerating where it is at. For instance, people using zfs on linux often have problems with running out of memory and so on, even on systems with very large memory.
Modifiable snapshots are called clones in ZFS. Everything you said about snapshots is possible with ZFS with clones+snapshots. Clones are just as instant as snapshots.
Runnin ZFS in linux is not good idea. That's why I'm currently using OpenSolars and maybe switching to FreeBSD in the future for file and db servers.
The problem with btrfs time estimate is that it takes long time for filesystems to become reliable enough that you can put important data on them. ZFS has crossed that threshold, it takes years and years for btrfs.
ZFS doesn't support an arbitrary graph of snapshots, even with cloning. There is a "zfs promote" command that can be used, but can only be used in certain circumstances. For example, I wanted to take the latest backup of a system and rsync older and older backups onto that backup, making a snapshot each time. I then wanted the snapshot of the newest data to be the "HEAD". I also wanted to gradually delete the older data. This setup was not possible with ZFS, because the child-parent relationships were in the wrong order. With btrfs it was simple.
btrfs can do things like copy particular files using copy on write, which is really cool. btrfs also supports offline deduplication, which isn't supported in zfs. This is very useful if you want to do the deduplication when the system is not otherwise being used and avoids the overheads of keeping hashes in memory all the time.
I think that 10 years behind is exaggerating where it is at. For instance, people using zfs on linux often have problems with running out of memory and so on, even on systems with very large memory.