The DX9 was a not-so-special spin off of the aggressively popular DX7. The DX9 wasn't a great synth, it had all of the headache problems associated with its DX7 relative. Difficult programming and limited MIDI implementation to name a few. Still, it is a 4-operator FM synth like the other DX synths and is capable of interesting sounds.
In the mid eighties when the DX9 appeared it was a welcome cheap alternative to the DX7. But current DX7 prices have nearly hurled the DX9 into obsoletion. Also, unlike the DX100 which is famous now for its gritty house-bass sound, the DX9 doesn't have any particular sounds that set it apart from other synths like it.
Demos & Media
Images from Synth Site.