An interesting thread and a bit spooky as I often wonder why they are not widely used in homes as well. If I could fit one at home, in the kitchen, I would and might do it when we do a revamp. My main reason is economy. About 5 years ago, we had a very dry summer and as a water saving measure, I collected water in a bucket (for my Roses and Hydrangeas)when I ran the hot tap. The bucket was 3/4 full before the hot water arrived. That means that I have 3/4 of a bucket of hot water in the pipework that just cools off after using expensive gas to heat it. The cost savings over a year must be significant.
When I was planning a self build a few years ago, my plan was to fit 2 of these. As has been said, one in the kitchen area and one for the shower. That is a good tip from Alan about setting the thermostat to the shower temperature rather than fiddling with the mixer tap.
The layout of my van is OK as regards the hot water apart from one big snag. If I want the heating ticking over on gas overnight, the hot water has to be on as well. The noise of it igniting regularly is annoying as well as wasteful of gas.