Ive done some extensive testing with the 7D and the built in transmitter, it IS really nice and there are some settings you cannot do with the ST-E2, like changing the manual flash of external speedlites from the camera. However…..one thing that is driving me crazy:

It seems that you cannot turn off the built-in flash to fire on the main flash. I am very familiar with how ETTL works (there are typically 2 flashes, a pre-flash as well as a main flash). Canon’s website even says you can turn the built in flash off to be a transmitter only, however, as of this writing (as well as the latest firmware update 1.0.9) the built in flash is still firing, Ill post the evidence later tonight.

If you know how to turn this off completely, I would really, REALLY, love to hear from you and how this is done. One small workaround I have found is to use the Ratio Control, turn built in flash to 8:1 ratio, and then turn down the exposure compensation to -3, – it will greatly lower the built in flash power, however, it will still fire.

The ST-E2 will allow you to trigger off camera flashes without ANY on board light coming from the camera. Im hanging onto mine.

If you are a 7D Owner, try a very simple test: Set your Camera to “Wireless Flash Only” and then take a picture in a mirror. Here are mine with and without (see above) an external speedlite.

This would also explain a very well discussed issue on some of the forums:

7D Wireless Flash Problem