I installed one of these about 4 years ago and installed it as you proposed in your latest post. Had no problems with operation or inspection.
(BTW, since this installation, I've strongly advised my clients to go with gas. The electric on-demand heaters require a huge amount of current and installation costs are disproportionatly high due to the wire, conduit (if required), etc. I've installed about 25 since, all gas)
I like to go with the subpanel at the device also with the main because of the high current use, though you can get small low flow units that service a small sink and use 30 amp, 120 VAC. Gas is greatly desired when dealing with these things instead of electric. These things can really break the bank in low power usage sites with demand meters.
I have a kitchen and bath on the other end of the house from the existing water heater and I am thinking about one of these or just a small tank to get some fast hot water. Right now you have to let the water run close to a minute, just to wash your hands if you want hot water. Plan B would be my "attic" water heater idea. Just a large diameter pipe running at ambient attic temperature and a heat trap at the water heater. The problem is finding large diameter CPVC pipe. Right now the water pipes run close to the ceiling drywall under the insulation, a northern idea.
Greg, I think you hit on the challenges we face with this issue.
I've seen the little under-sink water heaters, and have not been happy with them at all; the best they seem to do is make the water slightly warm. Half a dozen installs, and I've yet to find one that I like. (These were maybe 3 gallon tanks, and not true 'point of use' heaters).
Nor am I all that happy with 'instant hot water' piping arrangements; that's a lot of pipe to keep warm. IF I went that route, I might try to put the circulating pump(s) on timers, for high-demand periods.
Solar and attic heat collectors have merit, but there is always that issue of winter freezing. Before you know it, you have some pretty elaborate controls - and they're of little help for that 6AM shower.
What HAS worked is having a small (maybe 20 gallon) water heater mounted in the attic, above the bathroom. You feed this from the hot water line; its' job is to provide you the first few gallons of hot water while the line warms up. With this arrangement, there is very little 'stale' water to drain before you are getting hot water.
I tend to lean toward that small heater in the attic too. I am getting ready to rip out that far bathroom and it shares the wet wall with the kitchen so the plumbing will be trivial. I will put a big drain pan in there so if the worst happens I will just be watering the grass. (I have a 55 gallon PVC drum I am willing to cut up) I am thinking I can get away with a little 120v model.
Fortunately freezing is not a problem here. The plumbing is all in the attic anyway. My addition made that area of the attic easily accessible with a pull down ladder and a big platform to work from. I shoved 2 full sheets of plywood up there while the framing was still open.