I have an eye-fi card, the classic X2 model (8GB, class 6). I bought it when very few people knew about it, and it roughly works as advertised.
Which means: fine, if you shoot jpeg; slowly, if you upload large images to the camera.
However the biggest problem for me is what is not advertised, namely that you need both an internet connection and a special software just to access the card settings.
If you want the card to connect to a new wi-fi network, you need to log-in into their utility software, enter network name and password, which are uploaded to a server, from which (I think) it downloads a new "mini firmware update" on the fly.
So you need to do this in advance, unless you carry with you some wi-fi hotspot (a macbook is fine: macs can create a wi-fi network), not to mention, the potential security problem: there may be a database full of network credentials somewhere...
Today, I noticed that there's an alternative technology, a card that has an http server included: clients can connect, browse thumbnails and download pictures.