If the cooling system is running right, you shouldn't need one. I do have several, mostly to test and to use for comfort. I have the Targus ChillMat, Thermaltake's Massive23 LX, and CoolerMaster's SF19.
the Targus's fans run quiet but offer low airflow, and they are placed up high, which is away from the air intakes on the bottom of the G73 series. They are powered off the USB. This unit is very comfortable in the lap for extended periods of time, as weight is evenly distibuted.
The Thermaltake is about the same size as the Targus, with a massic 23mm fan, which runs at a low RPM off the USB. Airflow is more evenly distributed. In the lap, the weight is mainly focused on the rear, making it somewhat uncomfortable, as the rear ridge is applying the weight to one's thighs.
CoolerMasters's SF19 is more spacious, has a USB3 port hub, but is powered from an AC brick as opposed to the USB port. The 17.3" is smaller than the cooler, and sports twin 140mm fans. These fans are standard case fans, so you can swap them out for other models if desired. The SF19 fits comfortably on the lap with even weight distribution, tho not as comfy as the Targus with its soft padding.
Furmark results (Ambient 20C) with 800/1450 OC on G73JW-A1
Temps: GPU / CPU Core 0
No additional cooling: 83C | 58C
Targus Chillmat: 83C | 58C
Thermaltake Massive23 LX: 82C | 58C
CoolerMaster SF19 (Max Fan): 76C | 56.5C (Idle temps: 33C | 38C)
As the stats show, only the SF19 generates enough air pressure to actually affect temps much on the G73JW. If the mesh cover had larger venting holes, I believe the cooling results would improve significantly, as I feel the small hole mesh is choking airflow of the twin 140mm fans. The included fans are also quite noticeable at max fan rate. It would be interesting to swap out the fans for some quieter fans of decent quantity airflow.
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