The Eye-Fi is good hardware. It's a magic SD card that you put in an ordinary digital camera to give it WiFi capability. With the Eye-Fi your camera will automatically and wirelessly upload photos to your computer, sharing sites like Flickr or Facebook, printing sites like Wal-Mart. It'll even upload video to Youtube. I bought it solely to physically avoid plugging my camera in to a computer; I'd been using the crappy iPhone camera for photos because it was simpler to upload from.
But what Eye-Fi really is is a marvel of tiny WiFi engineering. Buried in a standard SD card the size of one fingerjoint is a WiFi adapter with an antenna powerful enough to work from inside a camera. All powered off the camera's battery. It's kind of crazy that cameras don't have WiFi themselves, but Eye-Fi's solution works pretty well.
But what's really exciting about Eye-Fi is what else you could do with it other than photography. GPS trackers. Cell phones. Game consoles. Basically any device that has a storage interface could be given WiFi capability. Sadly, Eye-Fi has focussed their product specifically on the photo market and is not hacker friendly, so you can't actually do anything other than cameras with the hardware. Too bad.
The basic Eye-Fi goes for $50; compare $12 for an equivalent non-wireless card. At that price the only drawback is that the Eye-Fi may shorten camera battery life. The other caveat is there's a confusing proliferation of products. It all seems to be basically the same hardware, but the fancier versions will upload to public WiFi hotspots and geotag photos. Between the artificial product differentiation and the closed platform it feels a bit like they don't know how to make enough money selling the basic thing. But that's OK, because the base product is great.
Update: the day after posting this, Eye-Fi upgraded its desktop software. The upgrade shut my machine down without asking permission! Even better, it tried to disable the anti-virus software while installing. Very bad.