Upload & Sell: Off
I have the 1d4 and eye-fi, and it works great with my iphone. I tried to use it also with my iPad, and it didn't work, and it messed up my settings with my iphone. I believe you can only setup the eye-fi card with one iOS device. Since I definitely wanted it to work with my iPhone, I deleted the app on both devices, reinstalled it on the iphone, and everything started working again.
I find that, pretty much every time I want to use the app, I need to go into the wifi settings on the iphone, select the eye-fi card, and then open up the eye-fi app. I use the card in the mode where it only transfers to my phone the pictures that I protect by hitting the lock button.
After you get the hang of it, and get it working, I find the card to be fantastic. It is a significant reason why I'm really hesitant to upgrade to the 1DX.