Hi timc -- yes, I'm a btrfs user also.
My btrfs filesystem runs on a pair of WD1002FAEX hdds, in the default configuration (mirrored metadata, striped data) with compression. It will be 2 years old at the end of this year.
root@imerabox:/home/don# btrfs fi df /mnt/DATA
Data, RAID0: total=966.00GB, used=771.86GB
Data: total=8.00MB, used=0.00
System, RAID1: total=8.00MB, used=84.00KB
System: total=4.00MB, used=0.00
Metadata, RAID1: total=37.62GB, used=1.56GB
Metadata: total=8.00MB, used=0.00
root@imerabox:/home/don# btrfs scrub status /mnt/DATA
scrub status for c112ed57-0e33-4d4b-82c9-5c55932c529d
scrub started at Thu Sep 6 13:49:07 2012 and finished after 3742 seconds
total bytes scrubbed: 774.98GB with 0 errors