I have something like 300G I routinely backup. This includes some large 12Gig images and other files.
I had been using ext3 on an external USB disk for part of the process.
Under ext3 doing "rsync -a /home /mnt/external_back/backup.jun.27.2011" it took 200 minutes.
I took the same computer, same external HD and reformatted it for ext4 (mkfs.ext4 /dev/sdd1). I then started the same rsync as above. The time was reduced to 170 minutes.
This is on Centos 5.6 x86_64.
I did notice the [ext4-dio-unwrit] process that starts up. One process for each core. I was concerned when they did not go away. But google says that is normal. :)
Jerry