I had one when they first came out (the original Pro). Used it in my 1DIIN and 1DsII as a cheaper solution for wireless "tethering" with Lightroom when shooting architecturals. I had a laptop outside of the room that was receiving the files and an on-site editor which was calling back adjustments/changes so that I could tweak the room and lighting.
It was slow, unreliable (wouldn't connect correctly most of the time), and full of bugs. As a software developer for the last 25 years, I spent some time with them on the eye.fi forum trying to work things out and they pretty much didn't do anything to make it better.
When I moved my cams to 1DsMkIII's, I tried it again and gave up. Ended up with the WFT-E2A and haven't had a single problem - fast and reliable. So, I gave the eye fi away to a friend to use in a P&S that they were capturing inventory/stock pictures for ebaying, etc. They tried it with an iPad, laptop, and a desktop and was so frustrated with the piece that they ended up taking it back out. The transfer times (10.1MP P&S) and unreliable connection (sitting the camera no more than 6 feet from the AP) made it still quicker to pull the card and dump it directly via a reader.
Granted, there is a lot of signal loss on a device that uses a printed antenna sitting inside a magnesium chassis (Faraday cage anybody?). But on a plastic P&S, that should have improved.
When it did work, I had it dump the files into a folder, which I set as a watched folder in LR. An import preset was applied directly (color temp, camera profile from Colorchecker Passport, noise reduction, lens profiling) and that was about it. The site editor would zoom in (on a Fujitsu tablet running Win7Enterprise), check out the details and flag/star the file for later "fine" editing on the desktop. The card retained the RAW files until all was good before it was formatted.
Now, I just capture the images the same way, either via WFT-E2A and FTP to the Fujitsu or via an ethernet cable the same way to a drop folder. The advantage to the WFT-E2A is that I can focus/fire from the TabletPC outside of the room so that in tight spaces, I can shove the camera all the way into the corner.
For studio portrait/fashion work and product stuff, I still tether directly via a 100' USB cable - the transfer is much, much faster than anything wireless.