I have both (well, I own 7 JD800s and 2 JD990s )
Love both for their basic sound engine - almost identical. The JD990 does have some added extras that are excellent.
The way I use them is quite particular, and for me, they are obviously an important part of my setup.
I have two room setup, and in one of the rooms there are 3 JD800s and 1 JD990 MIDI'd; while in the other there are 2 JD800's and 1 JD990 MIDI'd. The other two JDs are for spares (though they are both perfectly working and haven't needed to plunder them for parts yet).
The JD800 is an amazing instrument for analogue emulations, but what give it great power is it's approximately 108 waveforms which are not like the multiwaves in the M1, T3 or SY77. Instead, they are like partials - snippets of a plethora of sounds sources - and as such they are far more useful in a wide range of scenarios, even to the present day. Think how useful a sawtooth wave is on an analogue synth - well each wave on the JD800 is about as useful as that - so it has a staggering sound design potential.
Coupled to the huge array of real time controls means that you can stretch your sounds to great depths and heights. At the drop of a hat you can drop a percussive sound 4 octaves, and then layer it with three other sounds, say voice based sounds, pads and bells. Or it can be 4 percussive sounds at different octaves or transposed, each with different filter, envelope, velocity and aftertouch response - all controllable instantly and in realtime - whatever you want. Each with its own set of controls, and an on-off switch to mix and match. It's like a realtime Omnisphere machine !! The JD800 liberates S&S / Romplers to the realm of realtime creation, control and performance. Though I don't own one, the closest modern synth to it seems to be the Nord Wave.
But because it's only 24 voice polyphonic, I like to use 2 to 3 of them to layer sounds and retain the 24 voices on each instrument.
But I also use the JD800 for what is, to me, the most stunning pad sound there is. Its based on the internal program number 27 which is three layered sawtooth waves and one choral sound. What I do is remove the choral sound, dim the filter cutoff to mellow tone and slightly detune the sawtooths, and then layer three JD800s and a JD990 with that same sound. So each note has 12 oscillators (3 x 4 JDs, per note). As if that were not enough, what I do is play all of them through an SY99 keyboard. That's because the SY99 has a non-spring loaded modulation wheel. Now, when I turn the mod wheel up about one third, it triggers the LFO vibrato on all 12 oscillators across the JD800s, and since the LFO rate is slightly different on each 3 voice (by default on that JD program) the sound thickens to become the most stunning, lush, non-localized pad sound. Finally, because I mellowed the cutoff and because the aftertouch response on the SY99 is so smooth, leaning in on the SY99 keyboard opens the aftertocuh filter setting on each JD800, brightening the sound to the touch. It's so rich and stunning. Feed through an exquisite reverb and you're in pure Vangelis / Tomita territory.
So the JD800 offers all this amazing potential - but you've got to work at it and find its sweet spot.
The ideal setup is in my opinion a JD800 and a JD990 - the layering possibilities, the JD800 realtime creation of sound that can then be brought over to the JD990 to add its extra capabilities, the ability to manipulate JD990 sounds via the JD800 controls.... its all amazing.
And - if you place a JD800 patch on a JD990 they can be layered - they are indistinguishable to my ears.
Finally, I have to say, while I also own and very much like the JV2080 and XV5080, I cannot recreate JD800 / JD990 sounds on them. I do not know why. Lord knows I've tried - because of their huge polyphony - but there is definitely a combination of factors from the particular sawtooth wave on the JDs to their filter and envelope characteristics that just give them a silky smooth feel by comparison to the JV's and XV's (which sound strong in their own right for sure but not in the same way).
Overall - using at least 2 JD instruments together massively expands the setup and is far and beyond the sum of the parts.