I don't really run into the electric water heaters much, but my friend had a problem with his recently. I guess his had two elements, and one burned out. So the 50 gallon heater wasn't quite heating 50 gallons of water, plus it was sucking a fair amount of juice since the one good element was trying to compensate.
I like the Rinnai's, I'm putting them in the houses I'm building. They cost us about $800 which is similer to a gas 50 gallon heater. Installation costs are the same, for us at least on new houses. On an old house it could be more because the on demand gas heaters use a TON of gas while they are on. Usualy you need to upgrade your gas line to 3/4 and put a higher flowing meeter on. But overall you do save money.
You also get a nice tax credit with some of the on demand heaters.
http://www.rinnai.us/tankless-water-heaters/