The sensor doesn't sense humidity -- it senses temperature. It basically shuts off when the temperature gets to much higher than boiling. The design is absolute genius because it basically shuts off when the water is gone (as long as there is water it can't go above boiling temps). This means that it will never burn or singe your food.
It also means how long it will cook will depend on how much water you put into the outer pot. Assuming you use the basic one.
The main thing about rice cookers is convenience. That and the fact that you can't really burn the rice. At worst you put in too much water and the rice comes out mushy, or too little water and the rice comes out with a raw center.
Short grained rice such as sushi rice must be soaked before cooking. Long grain MUST NOT be soaked, or it will come out mushy.
The fancy new ones that are several hundred dollars have timers. The idea is that you preload it and set the timer, and wake up or come home to fresh cooked rice. The time that the rice sits in the water soaks it so it's not good for long grain rice fans -- it's designed with the Japanese short grained varieties in mind. I personally prefer long grained in general so I don't go for the expensive new ones. Besides, the basic rice cookers with an inner pot and an outer pot are highly versatile. I use the inner pot directly on range top to sautee the ingredients for pilaf and stick it in the rice cooker after adding the chicken stock. Saves me from having to baby a simmering pot. It will even keep it warm if I wanted to do that first and handle other dishes in the meanwhile.
You can also use the basic ones to steam just about anything you can fit into them. When you buy it, it will typically come with a rack (mine was aluminum, so now I use a cast iron trivet instead). Stick what you want to steam in a bowl or plate, put it over the rack, add some water to the outer pot, and hit the switch. It will be done in twenty minutes.
Edited by Capsaicin - 2/26/11 at 4:00pm