To add emphasis to what Jon wrote, btrfs is still experimental and whilst
the developers are going to try and not change the on-disk format they are
not making any guarantees that they won't do so if they have to.
Do make sure you have read the btrfs wiki first and understand the issues
(especially the ENOSPC one, which can bite even when it looks like there is
space free due to the way btrfs functions internally).