I had a formatter failure in the 2nd year of ownership, but since then it has been quite good (Z3100 44ps, bought new in 2009), I bought a used 24" z3100 non-PS to keep it company in 2011.
Both have been rock solid with a few minor issues (cleaning capping station and dust bunnies), a few cartridges have gone bad (~4 saying they needed replacement before empty), 2-3 heads have gone bad, 24" needed a belt. For 2 machines over 5-7 years, those costs have been very small and minor compared to the terror of the formatter failure so long ago.
For the formatter failure I think the filesystem went bad on my unit, so I always delete jobs from the HP after they are done, to minimize the risk of any large files overwriting into a critical area. It may be the equivalent of waving a chicken foot at it, but I haven't had any issues with the formatter since.
I print infrequently so it can be months before the printers see a job, depending. They stay on and self clean for months, but after a few weeks they often go into a loop or coma if I try to wake it up manually. Basically I just turn it off and back on before I start a job, but nothing more than that to keep things going.