In my six years of Z3100 ownership, I recently calculated that I have run about two football fields worth of fine art prints through my machine, and paid about $30,000 for ink (probably $1000,000+ for substrate). The accounting is a little difficult because I'm on my third formatter. One was replaced under service contract, along with over half of my printer's guts (overkill, but I didn't complain, especially since the tech had to fly here, rent a car, and It was all covered by my contract).
In addition, on my list I've replaced a few things myself, after HP service left Hawaii altogether. Two belts, the right side service station again, a small nylon gear in the paper transport mechanism, and a formatter card were on my nickel.
In the last month, I finally had my first printhead failure of the type I've read about where the ink colors mix. First, I started having ink starvation problems in just the light cyan channel after 4' of printing, ruining two large canvas prints. Then I was only able to print a few inches before the light cyan gave out each time. Finally, I did a diagnostic print and saw mixing of the light cyan and magenta colors. That printhead was on me, as it had 4,000ml fired through it and was out of warranty.
Strangely enough, only ten days later I got an error message to reseat a printhead. Shortly after than, it refused to print. That one was still under warranty, and HP replaced it for free immediately.
My laundry list might sound bad, but I actually am still thrilled with my decision to buy this printer and consider the failures relatively minor for the usage I've gotten out of it. I have no doubt that if I had purchased an Epson at the same time that I would be tens of thousands of dollars poorer in ink purchases alone.
I'd be happy to continue carrying the HP torch for some time, except for the fact that they seem to have abandoned 12 color fine art printing. They are too busy challenging RIM for the most poorly managed tech company, I guess.
