It is difficult to understate the importance of the Disk II controller. Cassettes were ridiculous. Floppy drives were awesome, especially two of them (which the controller supported).
Kind of the LaserWriter of its time: a very profitable peripheral that made the whole platform work.
Cassettes with automation, the way, say, the Epson HX-20 does, aren't bad - they are slow, but can do random access. That said, the Disk ][ was very much ahead of its time. A bit unfortunate the Apple ][ didn't have any sort of built-in timed interrupts to make the firmware work at higher clock speeds - this locked the II to 1MHz for far too long.
Kind of the LaserWriter of its time: a very profitable peripheral that made the whole platform work.