I read that the canon 600ex rt has some face detect eTTL magic. Has anyone found this to provide real improvements? I'm shopping for a second flash and want to know if this is worth taking into consideration.
Face detection has to do with focusing and is performed by the camera, not the flash. Whatever face detection features your camera body are capable of will determine the performance of the camera/flash system in this regard. With most bodies this would require using Live View to use the facial detection features. The EOS 1D X has an advanced metering system with RGB exposure meters rather than the usual monochrome metering sensor. This metering system can distinguish different colors/tones, detect typical flesh tones, and adjust exposure accordingly based on metering via the viewfinder as well as by using Live View Face Detection. The EOS 1D X uses this system to recognize skin tones when using PD AF so it is quite possible that when coupled with the 1D X the 600EX RT can adjust exposure to match skin tones without shooting in Live View (see the fourth bullet point here: http://learn.usa.canon.com/resources/products/eos_1d_x/eos_1d_x.shtml).
Once the camera has selected the point of focus it does communicate distance information to the flash for use in figuring the needed amount of flash power. Although the primary factor in figuring the power is the amount of reflected light from the pre-flash recorded by the area of the camera's metering sensor associated with the selected point of focus (which may have been selected by the camera using the facial recognition feature), the distance information is also used to allow compensation for highly reflective surfaces that would otherwise fool the metering system during the pre-flash measurement.
The camera body handles metering, not the flash. The 600EX-RT has no face detection feature as it doesn't even do exposure calculations. When using E-TTL, the camera triggers a pre-flash, meters the result of the pre-flash and calculates the flash power based on that metering. What could be possible is that your camera itself may meter based on where it detects faces, but that would be a camera body feature.
The only other thing I could think of that you might be thinking of is that the 600EX-RT has an IR illuminator that projects a near-IR focusing grid that can help focus while in dark environments. This might also help the face detect feature of some cameras that support it possibly since it projects a known pattern on to the subjects.