I have one and like it (made by Jason International). The specs call for the air coming in to be at least 70-degrees - it moves a lot of air, so you need a vent or something to let it in. They don't have a heater, per se, they rely on the fact that the air is being compressed, and therefore is concentrating the heat (like a bicycle pump's outlet gets hot when pumping up a tire). It does feel cooler than the water, but is intended to only slow the cooling of the water, not heat it. I like a hot bath, and it's still comfortable after the normal, 20-minute cycle ends. I just start with the water hotter than some people.
Much easier to keep clean than a whirlpool, but not as vigorous a massage.