It's old, it's noisy but hey, it sounds great. Not a device for imparting ambience to sounds (although the reverbs are quite nice and very controllable) but more a genuine effects processor. You can bend sounds in ways that weren't possible using any other effects box at the time of its release. It's looking a bit like old hat these days but it's still pretty capable and the built in parametric and graphic EQ section is a doosie.
What's it made of, then? You basically select a configuration from one menu and adjust the parameters of all of the effects in that configuration using other menus. Finally, you use the "mix" menu to decide how these effects are going to be routed, their levels and so on, before they finally hit the stereo outputs.
The MIDI implementation and degree of control stands up superbly today. You can control an almost bewildering number of parameters in just about all of the configurations.
The main caveat is that, if you get one, treat the power supply very carefully indeed. There are horror stories regarding the cost of replacement units.
The other caveat is that Jonathan's QuadraVerb died a sad, lonely death recently. He's now using his Yamaha A4000 as a processor for gear without its own internal FX.