I have both a 5D3 and a Eye-fi X2 Pro, and they work flawlessly for JPEG transfer to an ipad or computer.
The 5D3 explicitly supports Eye-fi cards - there's a menu option available when you plug one in. It lets you enable/disable the wifi transfer and view the connection info. You cannot change the connection details directly from the camera, though. Also, on the shooting info screen (hit info while you're shooting) it will show a ))) icon whenever it's connected and transferring.
I have only tried it on JPEG, but RAW should work as well, and it might be great for shooting in a studio. It has an option that effectively turns the card into an infinite storage card (as long as it can keep up with transfers).
No word yet on how it affects battery life.
To answer a few issues I've read here.
It is quite easy to set up. You start by putting the SD card into your computer and initializing it. You teach it what networks to use, and what device to transfer the files to. (You get separate options for JPEG, RAW, and video files.) The destination has to be set before you start shooting, but the iPad/iPhone can tell the card to send photos to it instead of your computer. It can't update many of the settings on the Eye-fi card except with your computer, but you can at least change the transfer destination.
The geotagging does work, but it's entirely dependent on your surrounding wifi networks. It does not use GPS.
No, you do not need constant wifi network if you're using the wifi direct mode. You load the Eye-fi app on the iPad and it will use the ad-hoc connection the card creates, as long as you're not using an existing wifi network on the iPad. If you are, then the card will have to use that wifi network as well... network+network or direct+direct, can't have it both ways. Also, the files only go through Eye-fi servers if you tell it to. ie. If you use the public hotspot transfer option or the Eye-fi branded photo sharing service. If you tell it to go directly to your computer, it shouldn't touch Eye-fi's servers.
Yes, the Eye-fi will work out in the woods, as long as you have direct mode enabled, and your iPad/iPhone is with you.
Overall, I don't think you'll be disappointed with the Eye-fi cards. They aren't quite the caliber of the Canon Wifi device, but it's only $99, so it's much easier to try out.