Xfs is great indeed. I still use it on my server. Especially the integrated fsck during boot is really nice.
The problem with Xfs is that it uses the head of the disc a lot and you can hear that. Ext3 is much more silent. Additionally you can resize ext3 partitions which is impossible with xfs. You can only increase partition size. Therefore I use ext3 for my root partition and xfs in the loop files for the small portage files.
The advantage of portage loop files are:
- (almost) no defragmentation of the main system
- much faster fsck for non xfs root file system
- easy to copy to other machines