I had both the DX7 and the DX7IID. I would buy it for $200 or so if in working order, but you should ask yourself if you are willing to dedicate time to learn FM programming. As mentioned above, FM is very unintuitive (no filter!), yet it can produce outstanding results, specially plucked instruments can be emulated way better than most analog synths. You will also need an editor to keep most parameters visible at the same time. Programming via the LCD is just too painful (you'll need to keep the whole architecture of the synth in your head to remember what you are doing!).
Cheesy sounds and all, the DX7 sounded very different to everything else at the time. Never got out of tune, had more polyphony than many other poly synths. On the other hand, the 4 operator relatives (to me) never sounded that good.
You also will find literally thousands of presets (although many are annoyingly repeated across banks). It might be worth getting the E! Grey Matter board if you can find one and a multi effects pedal.