Here is my experience. Harware 5 stars, software 2 stars. Average 3.5.
Installation file is 21 meg, hardly what I would describe as a compact file (per the manual). Installation offers updates to both software and hardware(firmware) Software is now 3.0.20 version, was 3.0.16. You also must then also allow Adobe Air Installer to access your computer, for non personal data only. Not quite sure what that does, maybe the web based geotagging. Then the SD card updated its firmware from 4.1010 to 4.1012. As installed the software is 12 meg total on a WinXP PC. Took 17 minutes. Reasonable.
Now taking pictures! It takes about 5 seconds for a 850 meg 7 mpixel photo upload. This was at about 50 ft distance from my N router. Right after the picture is taken it appears on the screen. By the time I took my test pictures outdoors and ran back inside to look, they were all there. Software allows to upload to any directory, but defaults to My Documents/ Pictures. Eye Fi helper software which does all this housekeeping can run in auto mode or you can manually start it. Geotagging works great, when you click on the thumbnail it shows a small Google map with the location of your photo. Once again this function is dependent on a Sky Hook recognized wi fi mac address hardware being in the vicinity of your card when the photo is taken. I loved everything so far. But..
Glitch one. And a bad one. Bad or test pictures are forever. A total of 10 photos taken generated a 1 meg log file, a 8 meg dated photo directory, a duplicate "spool" directory of full size photos, and a cache directory with thumbnail snaps. Tons of overhead. Should not be a problem to clean up. Wrong. Deleting the cache directory manually per the web based help file still kept a "palm tree" blank cache image in the eye-fi browser, with a now non existent file name below it. Why is this not a part of the software. I am now littered with palm tree thumbnails for all the pictures I took as a test and have deleted. Why? And why a duplicate "spool" directory after the data transfer has been completed?
There are lots of postings on the Eye Fi web forum site about this being an issue dating back to 2009, but between using eye fi helper, eye fi manager ? (web based additional software I had to add), and eye fi center I still could not find the elusive "delete upload history" button. It is supposed to be the answer. Seems like a common problem. So far, get this, the solution is to delete your account with Eye Fi, and then create a new one. I delete about half the pictures I take each time I shoot, and I have to do this every time???
So the hardware works great, after a short software installation. But the software is missing critical components. So this is only a 3.5 star review. I will upgrade it if, and when Eye Fi improves their photo manager software, or simply allows it to do the simple task of wireless transfer with geo cache data added.
Updated 4/23 If you check my comments file you will see that the Eye Fi folks were very kind to respond to my concerns and offer some additional help. Once the suggestions are incorporated into the actual software (as promised) so other do not have to struggle, I will up the software score. My suggestion is similar to what other software vendors use. Have a "simple" interface and an "expert" interface, such that the user can decide what level of control to exercise over the software. For example when on the road, without wi fi, the capability of turning off the cards' attempts to communicate needlessly and hence conserve battery power. Some newer cameras such as Canon T1i (T2i?) can do this in camera menu.
Update 5/2 The software is now up to version 3.1.2 and the delete local thumbnails is easy to find. After I hit that button, the spool file also seems to be gone. Some quirkiness in using the thumbnails to see the location on a small Google map - clicking next appears to confuse the program if all the pictures are not uploaded yet. Today I used the supplied USB dongle to transfer the pictures (it still uses Wi Fi, would have thought that it would detect that its plugged in and use the USB). Took about 5 min for 40 7mp 1 mb shots. Geotagging error just driving around town is relatively small, about 200 yards. Interestingly enough, the pictures that stay on the card are not geotagged, only the ones that were transferred wirelessly are tagged, so do not overwrite the tagged auto shots by a manual transfer into the same directory, as file names are the same.
Update 5/12 ALthough not recommended by Eye Fi many users have used a SD/SDHC to CF adapter in order to use the Eye Fi with their DSLRs that only take CF cards. This was mostly sucessful with the previous Eye Fi cards. Unfortunately neither I nor anyone else that I have seen has been able to use the 8Gb X2 cards with the adapters. Certainly not in Canon cameras.
Update 5/29 I have just gotten a Fuji XP10 waterproof camera. It has a metal body, but it still manages to transfer pictures to the computer wirelessly at up to 25 ft from a router. The card is not totally compatible though. The second you hit playback on the camera, you get a card error. The only way to recover is to format the card in the camera, which means you would have lost all the pictures taken up to that point if on the road. So do not assume that because the camera and the card are both SDHC that they will play nice together. Check the EyeFi web site.
Update 11/27 Yet another firmware update to 4.2120. The website warns that if you are running version 4.1XXX then you may get an error message and have to retry. Well I was running version 4.2001 prior to the update. And I got the error. It is somewhat humorous, starts with "Breathe in...." Reinserted the card per the instructions, this did nothing, you have to re insert the whole Eye Fi adapter. This time it offered to update current 4.2120 to a "new" 4.2120. Supposedly the standby power consumption is significantly less.