I was a big fan of the DX7 and one of the first in my country to own one - I bought it before it was available. And ... I still got it.
The big deal was that it had FM-synthesis. An amazing clear sound very different from other synthesizers, so it was never a replacement, but an addition. Often the DX-7 was used as a replacement for electronic pianos and such. But it had a distinguished sound.
Before the DX-7 I had a monophonic ARP Pro Soloist. But I got tired of it, and wanted polyphony. And when the DX-7 came out it not only had 6 oscillators that could be patched differently but it had a 16 voice polyphony which was awsome at the time, because most synthesizers had a 6-voice polyphony. (If it were analogue it would have needed 16 x 6 oscillators!)
The DX-7 could be used as a synthesizer as you can hear in this clip by Casiopea, my favorite group at the time (they still play though). http://nl.youtube.com/watch?v=VqPc7-GjCrA
keyboard player Minoru Mukaiya in action.

[This message has been edited by drdalet (edited 10-09-2008).]