Pros: Simple technology that works. Using only water you can keep your pillow Cool all night
Cons: Your pillow is only as cool as room temperature. Not very effective in hot temperatures.
Make no mistake, the Chillow (or "Chillow Pillow", as I affectionately call it) does what it says, it gives you that "cool pillow" feeling all night long. But this is no magic piece of technology, nor does it use a mini air conditioner inside of it, it doesn't "make" cold and won't turn your bed into an ice palace.
There are many concerns/complaints about this device "not working" and it's usually based on a misunderstanding on what it is supposed to do. Do you like how your pillow feels when you first lay down, refreshingly cool on your face/head? Constantly find yourself flipping the pillow over every few minutes, just to get that "cool side" back? The Chillow is an insert for your pillow, fitting between the pillow itself and pillow case. When inserted, your pillow's face will stay cool, all night long.
Before you can comment on the product, it's useful to understand the simple science behind it. Human Body temperature is around 37C (degrees Celsius). Typical room temperature is around 21C. Your pillow, when you first lay down is at room temperature, which is around 15 degrees cooler than your face. This is why the pillow feels "cool". It's not actually cold, it's just significantly cooler than your own body/skin, so it feels cool to the touch. (Much like metal, at room temperature often feels cool upon first touch).
The problem is once you put your face on the pillow, heat from your body starts to warm it up. And pillows are bad transferrers of heat, so your body sends more heat to the pillow faster than it can get rid of it. In fact most of the heat stays trapped on the pillow's edge, which is why if you turn it over the other side is cool. Very quickly the temperature of the pillow's side gets pretty close to that of your skin/body. There is no longer the nice difference, the "cool" sensation is lost and in fact the pillow may feel uncomfortably warm.
Enter the Chillow pillow. It's simple task is to keep the pillow at room temperature for as long as possible, thus maintaining that "cool" feeling. It's job is basically to act as a "radiator" to get rid of the heat from your face. The magic technology the Chillow uses, is simply "water". Water is a much better transferrer of heat than your standard pillow (which is often filled with feathers/down, which we put in jackets to trap heat in our bodies!). Water sucks the heat out of your face faster, since it can flow it transfers the heat throughout the whole Chillow device, and it lets go back into the air quicker. Sleeping on a pillow made of water would be the best solution to this problem, unfortunately that would tend to get you a bit wet.. So Enter the Chillow, basically a container to "trap" the water in a form that can fit inside your pillow, while trying to remain as comfortable as possible.
The reason it is filled with foam is to help give the Chillow some rigidity in it's shape (imagine the difficulty trying to sleep on a water balloon), and to act as a sponge to trap the water so that one end never goes "empty". The foam is light enough that it doesn't impede the flow of the water too much, so that heat can still properly move around.
One thing to note though is that the Chillow is still not a perfect radiator. It does not get rid of the heat to the air as fast as your head/face adds it to the pillow. So over the course of the night it will progressively get warmer. The company touts this as a 'feature', in that it warms to wake you up, but it's simply an unavoidable result. That being said it typically lasts the better part of a night, and is coolest when most needed (during the start of sleep).
Filling the device is not nearly as "serious" as some would have you believe. You do want to try and eliminate as much air from the Chillow as possible, as air is not as good of a heat transferrer, but it's relatively simply (tilt the pillow, so the air rises, then squeeze/smooth it out a bit to force any air bubbles out). The more water you put in the device the longer it will stay cool (more water to absorb the heat), however the less comfortable it will become as the extra water not absorbed by the foam will slosh around and bunch up (which is why the foam is there in the first place). But this is really a minor issue. I would recommend using a little more water than suggested, and if you find the Chillow too cumbersome, then removing a bit of water until you don't mind it.
Some simple upkeep is required. You should clean it regularly (oil from your head/hair/face may seep through your pillow and can react with the viny). Also keep it out of direct sunlight as the thin plastic can react and lose some of it's softness. From time to time you may want to change the water to prevent it from becoming putrid.
One important thing to note though, is the whole point of this device is to keep your pillow at room temperature. Pillow's only feel cool when room temperature is reasonably lower than body temperature. If you are in a really hot room, the chillow will be the same temperature and thus will not feel cool! If your room is 30C or more, the Chillow is just not going to feel that cool. It will be cooler than a regular pillow, but in serious heat it loses much of it's effectiveness.