Its usually the quality of the water that damages these or any other units. Due to water hardness If your in the South Island these units (In the south Island hot water cylinders are usually marine grade stainless steel) tend to corrode much quicker then the North Island. So really if you want to look after it as much as possible install a water filter/strainer before the unit to get the max life. Also check what the warranty covers. Generally speaking I have only installed 3 of these in the last two years, so have not been in long enough to comment. At this stage (2 years later) I have not had any complaints.

Regards Plumber