Was simple.. ext4.. tempted to try btrfs... :p
Give it a try perhaps on a virtual machine. I've seen it come a long way just over the past two years. While I wouldn't yet run anything mission critical on btrfs, I think the day is coming before too long when it becomes the filesystem of choice. I believe SuSE may be headed there already, making it the default.
Its snapshot capabilities are just so simple and well-designed. I dare say that it may be better than ZFS in some ways.