I recently purchased a second one of these - they're relatively inexpensive now. I really liked the one I got for a point-and-shoot camera, and wanted to try on in my Nikon D80.
Setup is pretty easy, though I thought setting up the direct mode access with the new Android and iPhone/iPad apps was a little confusing. The software is OK, could be a bit better - the EyeFi helper app keeps crashing on me. My D80 also seems to be unable to complete and upload of a large batch of photos (about 100 JPG high resolution). I've even set the power settings as their support pages suggest. Sometimes, you just have to plug the card into a computer.
The are where it really shines, though, is the ability to instantly put pics on Picasa or other sites to share with other family members.
Find it here at Amazon, about $40 as of this writing. Size is pretty unimportant since the card has a feature that deletes pics off the card after uploading once you've hit a user-specified capacity. That's pretty cool.
Overall I'd give it a reserved recommendation. The software needs some improvement.