The benefit to filesystem journalling is that if your computer crashes while it's writing to your hard disk, the hard disk directory is less likely to become corrupted; Disk First Aid need only scan the journal, not the entire hard disk, to verify filesystem integrity.
The downside is that it slows down disk access. A LOT. By about 20%, in non-MacOS X Server configurations.
Generally speaking, filesystem journalling is only useful on server systems.
