My experience with the XR90...
My fiasco with the Rheem XR90 began back in November when I decided to go a bit greener (and pay a lot more) for it instead of going with a much cheaper alternative for a tank water heater.
The first XR90 was installed professionally and found to be defective. After several hours on the phone we were told that there were no parts available, and that the only way to fix it is to send an entirely new unit.
So no hot water while I waited for XR90 #2 to arrive, and then a complete uninstall/reinstall of the defective unit with the new one.
Unit 2 is intalled and, lo and behold, ALSO found to be defective (this time 2 parts were not functioning correctly!). They offered to take parts off the first one that worked to make the 2nd one work, and I said "hell no", so they offered a 3rd new one.
I was given a very slight discount for my troubles (half of which was taken by the installers because their time was wasted as well), and a 3rd unit was ordered (again, no hot water for multiple days).
The 3rd one had no immediate defects after installation, but it's a very inconsistent water heater, and frankly, I'm not sure if I'm going to keep it. Why I allowed a third attempt is beyond me...
Anyway, one has to question Rheem's quality control (if they even have it). To get not one, but two defective water heaters from the factory says something. I've attempted several times to get an official response from Rheem over this mess, but have been largely ignored.
Think twice about getting this unit, or a Rheem system at all.