The volume they can supply is often limited, so if you are filling a tub, you may have to wait and you may have problems if you have two hot water taps running at the same time. They raise the temperature of the water a fairly fixed amount, depending on how cold your input water is in the end of the winter, it may not raise it to a decent comfort level. My inlet water temp at the end of the winter is barely above freezing. Most will raise the inlet water about 70 degrees, then add the loss of a long run of pipe, and you may have tepid water for a shower. It takes significant wattage to provide a large volume of water since it isn't in the device very long - you may have to upgrade your electrical service or live with a smaller volume of water. If using gas, you would need to provide combustion air, as a typical basement doesn't have enough air volume if you read the specs (for a decent sized heater - especially if you have a gas dryer and furnace (not direct vented) in the area). Many people ignore the volume requirement when determining if the thing will fit - they only look at its actual size and say it will fit where I want it. This can be a deadly mistake. Course, if you can see daylight through the foundation, it may not be an issue! The storage losses are way down, since there essentially isn't any, but if you get a well insullated storage tank, they may only drop a degree or so an hour, and you'll probably only have to replace it, never repair it. Don't expect the life or lack of maintenance or repairs to be the same on a tankless - they have more parts/sensors, and the heat exchanger gets less efficient as it ages. My unprofessional opinion. I'm going to replace my system this summer...