I've got the big natural gas tankless whole house unit. It is rated to run at least 2 showers. It does that even with our winter ground water temps around 50F. I can fill a regular bath tub hot water full on.
I think they are great units. They last much longer than tanks, take up much less space, use less energy and you never run out of hot water.
Just a couple of downsides.
First we have a large whirlpool tub with an extra large tap. Full on exceeds the capacity of the heater and the water coming out is cool. However previously with a tank the tank was empty before the tub was full so that was not a good solution either. Instead we just fill the tub with the valve only partial open.
Second you can't dribble the hot water like you might when washing dishes. The tank shuts off when the volume drops below a certain amount. It basically requires the valve on a sink be half open to operate.
Third if you do a dumb thing like leave the hot water running while you leave the house it will keep the water running hot the entire time, spending much more in gas costs that a regular tank.
The city had shut the water off to the house for some water main repairs one day. When my wife couldn't get hot water for a shower, she left the valve open instead of shutting it off. We left for the day and came back hours later to a hot steamy shower still running.
to Mike Flynn: The unit we have would heat your water up enough for a shower even with 38F input. However you wouldn't be able to run two showers like we can down here in temperate climates.
Craig