My .02,
Just like everything, they have their pro's and cons.
In colder climates, where the incoming water is colder(duh), the gallons per minute that the unit can heat, to a said temperature rise, is the toughest part.
Here, in WI, if the incoming water is 50 degrees, and you have it set to 120, your looking at a 70 degree temp rise.
The largest Rinnai commercial unit will provide 8 GPM with only a 50 degree TR, and thats burning 237,000 btu! (a std. home tank water heater burns 34-40,000 btu).
The largest Bosch unit I've seen would be around 6 GPM, with 199,000 btu and a 50 degree TR.
The best electric units I've seen are more in the neighborhood of 4-5 GPM with a 50 degree temp rise, and it uses 150 amps to do it, so a 300 amp service min., would be needed.
To put that into perspective, the valve for tub filling should flow from 12-18 GPM, shower heads run at around 2.5 GPM(if you dont pull out the restrictors), faucet aerators around 2 GPM, laundry WM 2-8 GPM.
Gas units will almost always need new gas lines run to them, many times, all the way from the meter, and I've had a few times where a new, high pressure(2psi) gas meter was needed. And venting is also many times heavier duty,
because while they save a good bit of money by not maintaining a tank of water all day, when they are running, they are not super efficient at all, the exhaust is very hot.
On installation, unless you are blessed to live in an area with completely soft water, make sure to have a valve arrangement installed so you can easily de-scale it, by hooking a pump up, and flushing a solution through it,
to remove the build up of mineral that naturally precipitates out of the water when it is heated.
Front load washing machines, and many HE washers add small amounts hot water, then tumble, then more hot water, causing the heater to cycle on and off, many times never reaching the set temp, sending essentially thermal morse code through the hot water pipes.
A new issue we're seeing is some of the really low flow faucets and shower heads wont flow enough to trigger them to run.
I have installed many of them, and most of the people I have put them in for, still like them. Almost all have mentioned "getting used to them", meaning not having multiple hot faucets running at the same time.
Personally, I will stay with my tank heater, as I have 3 daughters, and my wife.