[Speargun]

A buddy put one in his new house about 3 years ago. The electric company had to come out and install a new transformer on the pole because with everything on, he was pulling around 350 amps. Once that was taken care of, the lights quit flickering when the hot water was being used.

My shower was within 15 feet of the instant on hot water heater and had a tendency to "pulse" hot water. I.e. hot, cool, hot, cool... It wasn't unbearable, but it was annoying.
His room was on the other end of the house, about 40' away, and didn't have the same problem. My guess is that the water temp had a chance to average out in the pipe and thus gave a steady temp.

For a "lake house", small shop, sink, or anywhere there is only an occasional use, I would say that they are a good option since it is only on when you need it.

If you have mineral in the water just pick up a whole house filter at Lowes or Home Depot to help catch most of it before it gets into you heater.