RE: laptop coolers...
Hi,
I have tried six different laptop coolers, both active and passive. Of course, it depends upon the ambient temperature of your room as to how much cooling you will get from these devices. It also depends upon the size and number of fans in the coolers, as they have anywhere from zero (like the passive cooler pads that absorb heat by "melting" crystals in a gel inside the pad), one 50mm fan, two 120mm fans, to one 220mm fan. I find that I get anywhere from 0-5C CPU cooling when using these devices compared to just the rMBP's fans alone when the machine is running under full CPU loads, at my air conditioned room temperatures.
Switon
P.S. Basically, I think Apple did a pretty fine job designing their cooling system including the algorithm for fan speed control, seeing as I don't find too much additional cooling using these laptop coolers. On the other hand, the ones that work the best appear to direct extra air into the vents of the rMBP, although the ones that work the best also direct a significant flow of air across the bottom of the rMBP too.