I've had them in various forms for several years now. I started with the original Eye-Fi card and have worked my way up to the Eye-Fi Pro variety and wanting to get my hands on the X2 Pro cards now.
I think you'll either love it or hate it depending on your work flow, how many pictures you end up taking and if you upload to any online services.
Generally I take a TON of pictures, 400-600 a day depending on the event, just JPG, no RAW. I upload them all to SmugMug no matter what and do very little post processing as part of my work flow.
I got the pro card because I occasionally switch to a much more serious mode and shoot in RAW and it affords me the ability to automatically send those pictures to the computer.
I like the automatic nature of the uploads, sure I could just as easily insert the card, copy the images over to a directory, launch an uploader and upload the pictures manually, but its so much easier to either just turn on my camera and let it do the work or just pop the card in and walk away, no intervention on my part at all.
Another nice feature of the Eye-Fi, depending on which card you get, is the ability to upload wirelessly from thousands of locations. Lets say your on a road trip and want to upload some pictures, stop at the nearest McDonalds and have a softserve while your images upload for free from you camera. Ditto for almost any AT&T hotspot across the country.
Anyway, like I said, you're either going to love it or hate it. I've bought 5 Eye-Fi cards over the years and have loved every single one of them. I know others who used it one time and immediately got rid of it because it just didn't fit into their work flow.