This is the best new product I have ever seen! This card works perfectly with many of my SD cameras completely rendering cables useless!!
First off, the registration. A quick and simple registration (which you must have an internet connection for) helps you register the Eye-Fi Card with Eye-Fi's server to prevent unauthorized users from using *your* Eye-Fi Card. During the registration process, you also set up your wireless network (with your personal wireless DHCP enabled router), which works with Open, WEP or WPA encryption. It sends a test packet to the Eye-Fi Server, and once it verifies it's arrival, BAM! Your wireless network is set up. The next two are setting up your photo sharing accounts, personally, I use Webshots, and also directing your photos where to save on your computer. After that, you slap your Eye-Fi Card in your camera, take a picture and wammo! The picture is on the hard drive and on the photo sharing web site!
With so many new features added in the past few months, like geotagging and hotspot access on the Explore Card, this has become a necessity for anyone who uses a digital camera on a regular basis. I have not noticed any excessive battery usage on any of my cameras, which are two Nikons and a Canon.
Now, there are additional features, such as e-mail and SMS notifications, so if I'm not near my computer, I don't have to check the Eye-Fi Manager to see if my pictures have uploaded, my phone will tell me so.
It's a great product, but, please, don't buy this card with the intention that it will do what it is not advertised to do, such as create a wireless connection for your PDA, transfer RAW and movie files, send pictures back and forth between your digital picture frame and your camera. No, only JPG files, folks. And it does it flawlessly!