We backup exclusively to disk and have never been happier. We've always hated tape with a passion, mostly because they've been so unreliable (tape-eating drives and media not readable on identical replacement hardware because of imperfect head alignment). We've experimented with a number of disk-based systems over the years and have now settled on two NAS devices, one on site and the other off site. (We have our own fiber between two buildings separated by about a block, so off site doesn't mean off network.)