While we use tarsnap for our server backups, it's not something I'd use for frequent desktop backups.
The main reason is that it doesn't have a daemon that watches file changes to keep a dirty list so it'd basically DDoS your desktop every time the cronjob ran to run the backup. Also for a desktop it's nice to have an interface where you can see the differences between backups.
The I/O thing is actually a problem on servers too. See if you can spot the point in this graph where cron.daily runs: