I've actually been running the system I'm typing from with Btrfs on the root partition for some time now. Since I always have an ext2 /boot partition, I didn't have to worry about this.
I still wouldn't recommend using Btrfs for / unless you have a separate /home partition, though: there is not yet any fsck.btrfs! In addition to a separate /home partition, I'd recommend having another Linux install with ext4 alongside it, so if something does go wrong with your Btrfs partition, you can boot a fully functional OS without having to reach for a LiveCD. That said, the Btrfs filesystem has handled several hard shutdowns without issue (there's some problem with my Nvidia graphics card that's been causing intermittent/unpredictable hard freezes, impervious even to the Magic Sysrq key).
The cool thing about using Btrfs for the root partition on an ordinary system (this is why I'm trying it) is for fast, transparent filesystem compression. I have an OLPC XO-1, and a bootable FSF membership card, neither of which has much free space. The traditional solution to such space issues is to have a SquashFS (read-only) root partition to save space, and use a persistence file to store changes. But if the changes become too great, the persistence file will reach the size of the partition that holds it, and you have to recreate the SquashFS image to start using the system again. While the compression factor isn't quite as good for Btrfs compression (it can do gzip and lzo), it's still very handy when trying to get a full system into 2GB, and it lets you have read-write access with good performance, too.