I recently purchased the 8GB Eye-Fi Pro X2 SD Card from Amazon US (about $100 delivered).
Basically the idea of the Eye-Fi card is that it is both a standard SD but that it also has WiFi capabilities which allow it to do a number of other things including uploading images via WiFi.
One of the new features of the new models of cards (and with firmware updates to some older cards) is something they call "Direct Mode" and basically what this allows is that instead of the Eye-Fi card only acting as a client which can join other existing WiFi networks it can now act as a WiFi hotspot so that other devices (like an iPad, iPhone, iPod touch, Android) can connect to it and have images (and videos) transfered directly to them.
There is a free Eye-Fi App in the App store which assists with the Eye-Fi card working with an IOS device. The app is however fairly basic however it is free - there is a paid for app in the App store called Shuttersnitch which works with an Eye-Fi and is apparently very good however it's an AUD$19 app which is reasonably pricy (for an IOS app).
There are a few steps for setting up your card and app on your IOS device however once done you can put the card in your camera and start shooting. The images will transfer wirelessly and directly to your mobile device. You can choose to transfer photos or movies or both from the camera.
The practical upshot of this is that I can effectively shoot like I'm tethered (with no actual cables tethering me) with my iPad (or iPhone, or computer) and the images appear on the iPad screen a few seconds after I've taken them.
It works with your other images too - If you've transfered images to your Mobile Device from the built-in camera or perhaps the camera connection kit or even email, the Eye-Fi will see al of the images on your Camera Roll. This is good for people that wish to transfer or backup their images to the Eye-Fi Servers.
Tip: Turn on Full Screen Mode - there is a "Full Screen" mode in the Eye-Fi App Preferences. Once you turn this on, your images will come in and not only display full screen, but automatically advance to the next shot as you take them.
Tip: If you don't care about seeing the movies you shot on your mobile device, definitely turn those off in the Card settings. Otherwise things will be moving along nicely and then you see a long pause as it transfers over a large movie.
The Eye-Fi App only allows you to share your images to your desktop/laptop computer or up to the Eye-Fi service (which i don't use). However, the good news is that the shots automatically go to your device's Camera Roll. Since they are in the native camera roll you can use them anyway you like and in any app you like on your device.
As I have a Nikon D7000 with two SD card slots I have my normal 32GB card in Slot 1 and the Eye-Fi 8GB in Slot 2. I've now set my camera to shoot RAW+JPEG and I have the RAW's go to my 32GB card and the JPEGs go to the Eye-Fi card and if I'm running the Eye-Fi App on my iPad or iPhone the JPEGs get transferred there. You can also enable an Endless Memory Mode where your Eye-Fi card will automatically free up space once your photos and videos have been safely delivered (so the card never fills). Personally I don't mind having a couple backups of my files (even JPEGs) in case my main card dies for whatever reason and I loose my RAWs before transferring them to my computer and backups.
Note that the Pro model of the Eye-Fi card I have also allows me to send RAW's to the card and have them transferred (to my iDevices, my Mac or to the cloud) however I havn't as yet used that feature - in which case I'd probably set my camera just to shoot RAW and send RAW's to both cards. I'm still planning on importing my RAW images directly into Lighroom via USB although the Eye-Fi does also support the ability for me to wirelessly Auto Import into Lightroom via the Eye-Fi Card if I wished - I may try this sometime as well.
Another feature which I havn't used which comes with the Pro card (an upgrade option with some of the other models) is Geotagging. Note however that this is done using Wi-Fi Positioning Service (WPS) technology to tag your photos with geolocation information so it's not true GPS, it's only as accurate as the WiFi positioning allows and it's only available where the WPS covers (mostly the US).
I'll post more when I've had more of a chance to use it for various situations.