i use Veritas 5 Netbackup for a bunch of servers. Disk is still pretty expensive if you need months or years of backups. We keep 6 months of backups onsite, send out one of the backups every month offsite and routinely restore production db's to QA and other testing. Sometimes we had to restore last month's backup because we needed some data into current production like when an archive script goes haywire and deletes data without archiving it first.

We have a disk based backup system for NT and MS Exchange from Evault but it compresses the data and has other goodies to minimize disk space. Avamar is good too, but expensive.

With tape we have a lot of backups on different tapes. With disk it is possible that you can have your RAID array die on you and lose all your backups. HP's new 300GB drives, we have had half of ours go bad in the last year. Once we had 2 die in the same RAID array in one week. Luckily HP send replacements out next day air and we rebuilt the array after the first failure and just in time for the second. Otherwise it would have been a 500GB archive database down the drain.