Laptop Coolers are not useful because you already have side cooling vents, but no bottom vents
What is a laptop cooler?
A laptop cooler is something placed under the laptop to assist in the machine's cooling.
They come in a variety of sizes and types. Laptop sized, notebook sized, even netbook sized, are available. There are also folding, sliding, even passive vs. active. It may also come with additional USB hubs and more.
A laptop cooler is supposed to keep your laptop cooler by dissipating heat from the bottom of the machine, either by conducting some heat away from it, or by circulating air across the bottom of the machine. Advertisement for laptop coolers often cites premature failure of internal components due to heat stress, and you would want to avoid it.
However, a recent New York Times Tech Blog entry (GadgetWise) stated that the only real reason to use such a cooler is for YOUR comfort, NOT the machine's usable life.
Heat transfer happens via three mechanisms: conduction (by touching), convection (circulating via another medium, such as air), or radiation (infra-red waves). So if a laptop cooler works, it has to occur through these methods. And conduction is the most efficient way to transfer heat, THEN convection, THEN radiation. Keep that in mind.
us examine how a laptop cooler really works when examined in detail.
Cooling by conduction
In vast majority of modern laptops, the bottom is made of PLASTIC. There are some that is made of aluminum, magnesium, or other metals, but those are few in number.
Plastic is NOT a good heat conductor. In fact, it is generally considered a heat insulator, depending on specific formulation.
Thus, cooling the bottom of a laptop made of plastic is just just barely helpful. It is like trying to cool hot liquid in a plastic cup by putting the ice OUTSIDE instead of inside the cup.
Cooling by convection
Convection is how the "active" coolers are supposed to work: by forcing air across laptop's "hot bottom", to draw away the heat.
Do you see any seams at the bottom of the laptop? No seams at all. There are not supposed to be any seams in the bottom of a laptop any way, right? So the air is not going through the laptop itself. The airflow is simply across the bottom, mostly made of PLASTIC, which does NOT conduct heat. It'd be like trying to cool a plastic cup of hot liquid, not by blowing on the liquid, but blowing on the outside of the plastic cup.
Cooling by radiation
Well, the color black actually absorb heat, but it is at the bottom, so it's not really seeing any light, thus, radiation is not a significant factor in either heating or cooling.
How laptop cools itself
If you look at a laptop you will find that it probably draws in air through the gaps between the keys of the keyboard, probably with an internal fan, or via conduction through "heatpipes", and vent the heat through the side vents. My Sony VAIO laptop has this feature, and it is several years old already. So this is hardly new.
Examine your laptop and feel where the hot spots are at the bottom, and where are the heat vents. It is probably not where you think it is.
Ever seen a thermograph?
A thermograph basically is a thermal photo, like those IR cameras shown on Mythbusters or Time Warp (both Discovery Channel programs). What if you apply one to laptop cooler review? Benchmark Reviews did just that.
I don't want to steal their picture, so I suggest you go take a look at their review. (Very nice review, by the way) Go to the "test and results" page, and you'll see a thermograph of before cooler, and after cooler. The hotspot in the middle is reduced SLIGHTLY, and both CPU and HD reported slightly lower temperatures. And that's a premium cooler (MSRP of $70 USD).
Laptop coolers, for most laptops, notebooks, and netbooks, provide much less benefit than you may think, because they simply draw heat from the wrong part of the computer, through barely-heat-conducting plastic.
Laptop designers are not stupid. They know that the bottom of the laptop is usually leaning against a desk, or pillow, or your lap, and they do NOT want heat to go there, since from there it goes nowhere. Notice the "feet" of that Toshiba laptop? The bottom does NOT sit flush to the surface. Most laptops are like that nowadays. They have side vents, which cannot be obstructed easily. And you don't need a laptop cooler to do that.
At $20-100 per pad, and the fan drawing power from the limited onboard battery run-time, AND one more item to carry, a laptop cooler's disadvantages simply outweighs any benefits it may offer.
If your laptop has clogged vents, or is already overheating often (older laptop, or desktop replacement, which run hotter), then you will likely need a laptop cooler. However, for laptops that are working fine, you probably do not need one.
You can always reduce heat build-up by using power-saver mode on your laptop, which reduces CPU speed, and thus heat build-up.
And if you really need one, get a soft passive pad, like the ThermaPAK HeatShift. Those sit flush to the bottom of the machine, no matter the "feet". HeatShift shows a 9.2 degree drop in average temp in a CNET test. Your results may vary.