Pretty good all considered
We installed a prodigy 3 from Bunnings in 2010 after old unit broke down on the weekend. We are a four person family but can have up to 7 with relatives staying. We always had good, strong hot water even though certain family members have very long showers!
Standard maintenance items are thermal relief valves and anodes every 5 years.
The gas regulator failed a couple of years ago which meant pilot kept going out and this was about $150 I think from one of the plumbing shops.
Ours was within a few weeks of the 7 year tank warranty when the tank started leaking, but Dux still warranted the replacement which I was pretty happy with. We had to pay for the plumber to do the install, but they fixed some issues with the previous install and upgraded it to the prodigy 4 star at the same time.
I can't comment on the issues others are having but the plumber seemed to think you should get 15-20 years from these units if you keep the maintenance up. For the life of the tank it is critical that:
1) the relief valves are regularly tested and replaced - these do fail and if they don't open properly they will cause tank to burst
2) sacrificial anode is replaced at 5 years. The anode is there to corrode instead of your tank. As soon as your anode is depleted your tank will start rusting from the inside.
I'm hoping the gas bill is a bit lower with the prodigy 4. Our gas bill has always been pretty high but we do a lot of cooking so can't just put it down to the p3.