For me initially the attraction of the iPF5000 was not having to swap blacks as one does with Epson's large format K3 ink printers.
Since that move I have nothing but praise for the 5000's image quality and reliability. During early December I printed some 1,700 11X17" prints in 25 days of almost continious use, without a head clog, misfire or feed problem. Quite remarkable.
The printer still annoys with its poor user interface and lack of accesible documentation, but otherwise its a honey.
The Z series printers do not have paper cassettes. There is only sheet feed or roll feed. Same as the large Epsons like the 7800 and 9800.
Also, you can't compare the cost of the iPF5000 and the Z printers, they are quite different beasts – different size, different features, different markets.
As for price, figure in the fact that they have a built-in spectrophotometer, a 40Gb hard drive and a web server, and you'll see that they are very compatatively priced.