I have a 5 Takagi tankless WHs installed at my house, office and at rental properties, and I've seen a number installed a home inspections here in Chicago. Eventually we got them all working well, and I'm quite satisfied with them, but we encountered a number of issues when installing them.
I ended up writing about this so much that I eventually put up a web page which discusses some of the "gotcha's" we have encountered and how to avoid them. It also has some information on calculating the payback of tankless heaters:
http://paragoninspects.com/home-insp...blems-faq.html