With the DAT tapes you can use the media about 25 to 45 times before it should be replaced. With the DAT drives and a daily backup you should use a cleaning tape at least once a month they are very susceptible to dirty heads. With other drives read the literature that comes with the drive about cleaning. To end the discussion on backing up your data a few alternate methods come to mind. With the size and cost of hard drives today a method gaining popularity is to purchase a second drive of the same size and on the designated backup day just do a xcopy of the drive to the second drive.
Advantages are cost, speed, and no special software needed.
Disadvantage is media is not removable so off site storage is not possible, and if drive space gets scarce to use this drive as a second drive hence no backup. The thinking on this one is that it is very rare for two drives to fail at the same time. Along the same vein are the larger removable media hard drives. Advantages is speed, removable media, drive can be use as a data drive with the change of a cartridge. Disadvantage is cost of media and current size restriction to about 2 to 4 gig.