I have been using EyeFi memory cards since I first saw them at Macworld many many years ago. I probably bought an EyeFi card within the first few months the company put the cards on the market. In total I probably own 4 or 5 cards with capacity going up to the new 16GB card. All of the cards have been reliable and the wireless upload with Geotagging of images had been fantastic.
With the Nikon D5100 I had no problem using the card. It was a little slow to upload to my computer but I didn't mind the wait. Now with the 24 megapixal D600 I am finding that the wireless download time of a RAW image is just unusably slow. JPEG images upload really fast but RAW is impossible. With 200+ RAW images taken on a typical shoot, it would take hours and hours to complete the download.
With the slow download, I am finding myself removing the card from the camera and putting it into the iMac's SD card slot and doing a drag and drop so I edit my images.
Just to be clear, I do not think the card itself is the main source of the problem. Home internet connections are great when downloading from the web, but uploads speeds are pretty bad. The EyeFi card uploads every image to the web and returns it to your local computer and that is just a super slow process.
If anyone has suggestions for getting a faster download of a RAW file from an EyeFi card, let me know. For now, I am probably going to drag and drop my images even if that means the file doesn't get a geotag.
I recently made a minor change to my wireless network and suddenly I have seen a speed increase in my Eye-Fi transfers. In the Wireless settings of my Airport Extreme I changed WPA/WPA2 to just WPA and suddenly transfer speeds jumped. Not sure why, but that is the only change I made to my network. The Eye-Fi card is working fast enough now to use it with the D600 RAW files.