Kingston HyperX 16GB (2 x 8G) 204-Pin DDR3 SO-DIMM DDR3 1600 (PC3 12800) Laptop Memory Model KHX16LS9P1K2/16

Pros: Very fast, runs cool

Cons: A little pricey compared to other RAM of lower latency/voltage.

Other Thoughts: Works great in my 2012 27" iMac. Increased the RAM to increase the performance of my Windows 7 virtual machine and I can definitely tell the difference. According to the Windows 7 (virtual) experience index, my RAM performance went from 6.9 with the stock Apple RAM to 7.8 with this RAM...almost maxed out! Also, even with doubling my RAM, the temperature of the machine seems to have gone down about 1 degree celcius...not much until you consider that there is a lot more silicon pumping out electrons at high speed.

Pros: Fast. Although I doubled my RAM, there doesn't seem to be a heat or voltage penalty as measured by a watt meter and software temperature gauge.

Cons: Price.

Other Thoughts: I was hesitant to get this since I didn't know if it would be compatible with my 2012 iMac, but it appears that it is. It immediately recognizes it as 1600 Mhz, but I'm not sure if it's running at 1.35 or 1.5V. All I know that it appears to be working fine. I increased my RAM mostly because of the virtual machines that I use. According to the Windows Experience index on my Windows 7 virtual machine, the Apple memory registered at 6.9 whereas this RAM registers at 7.8...almost a full point increase. Not sure if that matters, but overall, my computer seems faster...could either be the extra RAM, lower latency, or both. I may just order a second set to max it out.