I wouldn't suggest it, for the following reasons;
You will need a 3/4 gas line to the unit. That can often require running the line all the way from the meter as there aren't many spots in the home where there is a 3/4 line to easily tap into.
You cannot use the existing w/h vent- needs a bigger one, usually involving removing the existing roof jack, and doing a little roofing repair.
You will need a 110v plug near the unit.
You cannot use a recirculating pump, like a little Grundfos, with a tankless hot w/h- all you will be doing is recirculating cold water.
You still have to wait for hot water, just like a regular tank unit.
Installation usually requires a building permit/inspection.
They are expensive. Plan on spending $3K to have a plumber put one in. If you need the electrical/gas line/ new roof vent, and are hiring it out, you could drop $5k total on the installation.
It has electronics in it that are not readily stocked on the shelf. You can't go down to your plumbing supply house and just grab replacement parts like a thermocouple for a regular tank heater. And because the stuff is not on the shelf, I have heard of people waiting two weeks without hot water waiting for parts to arrive.
The units don't kick in until a minimum flow demand is reached, like 2 gpm. You don't necessarily have hot water for low-flow applications.
The only application where it might make sense is you have like 9 teenagers that like to take two 30-minute showers each every day.
I have a 100% success rate of talking people out of them as a contractor. Let me know if my string remains unbroken.
