Philip Johnston at HD Warrior posted a short article about using it last week: Testing the Canon WFT-E6 Wi-Fi dongle with the C300 (Updated)
Downside is the slight delay in picture reaching the iPad it updates about 1fps, Tudor has asked if this could be used as a directors hand held remote monitor, yes if he or she is only using it for composition, shot length etc. Lip sync or moving shots…no ! I will ask Canon if the lag can be improved upon.
B & H lists the Canon WFT-E6A Wireless Transmitter (note the "A") for $499, and only mentions the EOS-1D X, though I think it's the same one that works with the C300.
And the 5D Mark II now lists at $2,199.