The Original Hott Rod
This unit has a small (very small) anode as part of the heating unit.
1) At 100 bucks a pop, you would be replacing it every 2-3 years (depending on water acidity) Faster if more acid, slower if more base.
2) I doubt you would break even on propane saved by the time you needed to throw it away and replace the heater because the anode was used up. (unless propane got a LOT more expensive.
3) The electric heater is very small (450 watts). Recovery time would be longer than the standard 1000 watt element of a regular dual power (propane/electric) water heater.
The Original Hott Rod
4) You would need to run BOTH the propane and the electric to get enough recovery time to take a long shower. Electric alone would not be sufficient to take a shower using more than 6 gallons (a full hot water tank) (or 10 if you have a 10 gallon heater).
5) Having to use both when showering means the biggest advantage of the unit is "no advantage" and you still need to replace it every few years as the anode is used up.
My review from just reading up on it is: "Forget it".