My only serious complaint about the eye-fi cards is that they do not support the fastest available connection. If I bother to take the card out of my camera and stick it in my laptop it is just inexplicably stupid that it doesn't take advantage of the 312MB (BYTE, not bit) per second transfer rate of the SDHC UHS-II interface. There is a 2.5Gb/sec interface on the card and it insists on forcing image data on a death march out the wifi interface to the router then back as if the goal is some Darwinian selection of the fittest bits, only the few that actually make the arduous trek are worthy of extracting from the camera.
If I go out for a day to some photogenic or videogenic event it can take 3 fully charged batteries to fully transfer the data off the camera (photos and videos that take less than one battery charge to capture!), which is the fastest technique but ultimately ends up adding meaningful incremental cost per transferred bit as batteries have finite lives and are not cheap (($50/battery) / (500 charges/battery) = $0.10 per charge, 3charges/transfer = $0.30 cost of transfer per busy day of photography).
The official recommended work around for that untenable situation is to pop the card out and stick it in my laptop. Great - the card is now line powered, but as laptops are RF shielded to reduce noise, the transfer rate is miserable in the laptop and while the wee "I think I can" wifi radio in the card actually manages to connect, the images are not downloaded even overnight. So the next day, half way cleared, you're back to burning battery cycles to prepare for the next day's photo fun.
The wifi connection works great for a few photographs, or shooting around the house or office with the card on the wireless LAN; that's a great convenience and worth money. I love the geotagging feature, it is also worth the price of admission to get geotagging in a camera that doesn't have built-in GPS. I was amazed that it fairly accurate tagged photos in places where the wifi really shouldn't be mapped, like on VBC. I love the flexible file transfer and directory naming options. I really want to transfer my images through Eye-Fi Center to get all those features. I HATE that I have to spend days waiting for the transfers to complete over a the few Mb/sec of residual data that makes it through the RF shielding of my laptop while the card ignores the 2.5Gb/sec interface microinches from it's stupid face.
And this lack definitely hurts sales, at least from my recommendations. I have many friends who've dropped money on nice 4/3rds cameras or have good SLRs and ask me about the eye fi card, and while I extol the Geotagging, I get to the point about the card being defiantly ignorant of UHS-II interface for data transfer and they lose interest. There are plenty of times that I give up myself and use a non-wifi card for the lower power consumption and use exif-sorter and geosetter to achieve the same goodness Eye-Fi Center provides while taking advantage of what is frequently genuinely 1000x better transfer rate through the SD card interface.
Fix this one stupid feature failure and the card will go from a "cool and clever, maybe worth the money" lark to "this is the card you must have with your camera."
(Also, I'd really like it if the camera stored the raw WiFi information used to geotag permanently with the pictures - that is record all MACs, SSIDs, and relatively signal strengths in the IPTC comments section.)