It has a 25* year warranty on the heat exchanger, which beats conventional heaters which typically have 8 year warranties. Of course, we got 20 years out of our Rheem 50 gallon heater, and it was still working fine when we replaced it.
So, how long will it last?

Edit: Yesterday we had 2 showers going full blast and there was no issue with sufficient hot water. It keeps up just fine.
Like Scooter said, they are greatly improved from when they first came to the States.
*Second edit: Their website says the heat exchanger now has a 25 year warranty!