These systems are VERY popular in Europe. And it has nothing to do with metric vs. standard measurements. :) (They still need hot water like 'us'.)
Anyways, they're becoming more popular in the USA. Visit Google.com and start perusing the links to see what you can learn. I don't have one in my home but the next time our unit is about to go, I'm going to look into it. I do like the idea. Yes, the heat is as instantaneous as a regular HWH. Well, maybe the 'wait' is a little longer since the water has to be warmed up on demand. The only thing I'd look into is that and whatever maintenance is needed. I can't imagine what needs to be done 'regularly' but you never know.