I have just received my Eye-Fi card and it does work between my RX100 and the iPhone.
The menu entry to enable/disable it is "Upload Settings" in "Spanner 2".
It does seem to lose the WiFi connection to the iPhone regularly, even though they haven't been more than a foot apart through all my testing. It then is a real pain to get the images to upload at all, you seem to need to take more photos, reconnect, etc.
It uploads the full 20MP images.
I turned off Raw uploading.
I'm wondering if turning off GeoTagging will reduce the power consumption?
I haven't found a way to enable selective uploading rather than uploading all images. This is annoying. (The menu item is not editable on the phone.)
It names the files as the next free IMG number, this is a pain as it's hard to tell the difference between them and iPhone pics when you want to delete them to save space (as they are still in the camera). My GH3 keeps the Panasonic filename which is much better.