I own 5 ZWs and wouldn't part with them. Probably 2 of them will be used to power lights and switches on my layout, but for running trains, in the last 6 months I've bought 2 brand-new Z4000s to power the trains, and am very happy with them. I think what I like best about them is the steady power when I hit the whistle button; unlike older transformers, there's no power surge that makes the train lurch forward.
For me, to buy a used Z4000 or anything with electronics, the price would have to be a small fraction of the cost of a new one since there would be no warranty.
John