Note that you really want to be running the latest kernel possible if
using btrfs;  since 2.6.39 there have been several major performance
fixes, stability fixes, crash-corruption fixes, which users did hit on
a somewhat regular basis.  Btrfs is not yet stable for the typical
user who just wants things to work, even when things don't.  I don't
know of any major distros that offer support services for btrfs
filesystems, for instance.

I'm not planning to run my whole system on btrfs just yet - but I was
keen to start running one or two test filesystems on a server that I
currently have running Debian squeeze.  Everything else on the server
has to remain as-is if possible.

Thanks for the feedback - I will start looking into the newer kernels
and see if I can use one of them with squeeze, or maybe I will just set
up a VM for btrfs

One thing I've already noticed in 2.6.39 (and both versions of the
tools) is that df results are misleading.  E.g. if I run regular df (not
btrfs fi df), I am seeing the same amount of available space for all
filesystems.  Is there currently a way to see space used by each
subvolume and snapshot and which kernel and tools versions might be needed?