I think it's a great synth. It's basically a subtractive synth with about 110 single-cycle waveforms (and you can get more via a waveform expansion card). Yes, the knobs and sliders make it very easy and quick to get around on, although there are still some parameters that you have to go to the menus for (mainly the effects parameters). Lots of modulation options; one thing I really have gotten a lot of use out of is the multi-segment envelopes. Two LFOs and you can offset them so that, for instance, when you modulate pitch by an LFO, you can offset it so that pitch does not go below, or above, the base pitch.
A patch can have up to four layers. In monotimbral mode, you have two groups of effects, an A group (lots of weird things like distortions, flangers, wave shapers, and formants), and a B group (chorus, reverb, delay). In multitimbral mode, you can only have the B group effects. The multitimbral mode lets you play up to five patches, via keyboard split or separate MIDI channels. There's also a "special part" where basically every note on the keyboard can be assigned its own single-layer patch -- handy for building drum kits or collections of effect noises. The layer and multitimbral controls that select what you are editing at any moment are a bit confusing at first; you have to read the manual and then play with them to see how they work.
One thing to know is that this is an unapologetically digital synth. It is not intended to be a VA, and you won't get much out of it that sounds like it came from an analog synth. It has its own sound, and actually sometimes the digital aspect lets you do some unique things. For example, the filter doesn't self-oscillate -- but since it can go to very extreme resonance settings without self-oscillating, you can get sounds out of it that you couldn't get from an analog filter.
The MIDI implementation is very complete, although it does use Roland-format sysex for the slider values, rather than CCs or NPRNs, so if you want to control parameters with an external knob box, that may be a problem. But you've got so many controls on the panel, why would you need an external knob box? There is a rack mount version of the synth called the JD-990, which has some additional features. If you have both, the JD-800's controls can edit most of the patch parameters on the 990.
The performance controls include a good version of the Roland combination pitch bend and mod lever. There are buttons for solo mode (basically a mono mode), portamento on/off, and a transpose whose interval you can set. (I keep mine set to one octave down.) There is also a set of four sliders called the "pallette", which can be handy for multi-layer patches. How that works is: You can choose a parameter for the pallette sliders to control, and then each slider will control that parameter in one of the four layers. During a performance, you can turn off all of the layer edit buttons so that touching a knob or slider or button selects that parameter but doesn't change it. The pallette sliders will then control that parameter.
So how does it sound? Well, as I said, it doesn't sound analog. Having said that, it can do some great smooth evolving pads with the multiple layers, some very cutting lead sounds, lots of things that sort of sound orchestral but are also kind of quirky. The filter is multimode and with high resonance it can be very cutting if you want it to be. Some of the waveforms are really out there and can do extreme noises, or add quirk to other sounds.
Things to watch out for: Mainly the keyboard. Nearly all JD-800s suffer from the aftertouch rot that was typical of Rolands of this era. Some also have had problems with glue delamination in the contact strips that cause dead notes or uneven velocity response. These can all be fixed if you have patience. It used to be possible to buy entire replacement keyboard mechanisms from Roland, but I think they have gone out of stock recently. Other than that, the JD-800 has no particular maintenance problems. It's a good idea to periodically get a vacuum cleaner with a crevice or brush tool and vacuum out the sliders. One other thing: it's big and heavy. Make sure that your stand or wherever you're going to put it can hold it.
Switches, knobs, buttons, LEDs, LCD screens, monitors, keys, mice, jacks, sockets. Now two joysticks!
