I've had Time Machine running since June 21 on a 498 GB partition of a 750 GB hard disk. My Intel Mac mini has a 55 GB hard disk. Currently, TM has used 420 GB of the partition. Runs every 15 minutes. Aside from the purring of the external drive (Maxtor), it's undetectable. I've yet to need anything restored, but it has the coolest interface of any piece of software ever made, so it's slick just to look for stuff.
It doesn't appear that there's an option for only weekly backups, at least from within the application itself. There could be some Terminal voodoo that does the trick, however.
My iTunes library is on a separate partition and not backed up. My iPhoto library is also on a separate partition that IS backed up.
Only issue was a power outage during a late-night backup. Had to delete an intermediary file then everything was hunky dory. Well worth the price of the Leopard upgrade alone.
mt