I've used a DX7 extensively for several years and just sold it and bought a TX7. I wanted to free up some space, but now I program all my FM sounds via DX Heaven for the Atari ST (picked up a 520ST for two bucks at thrift shop). The DX7 has possibly one of the worst interfaces ever designed. If you didn't have a manual, I don't think you would ever figure out how to set the sysex perameters. The MIDI button isn't even labelled!! This thing went through some serious cost cutting before Yamaha released it. Pfff... assholes. FM synthesis isn't really that difficult to grasp, but using the DX7 front panel to program is like painting an entire house through the mail slot. Really. Download an editor with graphic envelopes and you can seriously pimp this thing. I sit down to program and go from industrial and EBM to smooth r'n'b with a stop at everything in between. I like it for very sterile, digital sounds. It's so inhospitable sounding, and all the microtuning possibilities can add alot of tension to your sounds. All you have to remember is that operators on the bottom row of the algorithm are carriers, and the ones on top are the modulators. As you increase the volume on the modulator operators, you get more harmonics, just like opening the filter on an analog synth. Sit down with a good editor/librarian program and it becomes clear really quickly. After a short while, it's easy to program AND have an idea what you're going to end up with. Just don't let Yamaha engineers rape your creativity with a ridiculously under-equiped interface for a very complex synthesizer.
Check out the download section at Tim's Atari MIDI world, and liberate your DX. Peace!