I have one of the 8GB models which I use with my Sony NEX-5. It is quite good in general, although there are a few fiddly annoyances I have with the way it does things. In particular, it uploads all the photos to a central server which your computer then downloads from, which can be kind of slow and uses a lot more bandwidth than it would if the card could just send directly to your computer if you're on the same network. Also, at least the Mac version of the software is kind of fiddly and reeks of "we didn't want to write our software for the Mac, we just ported it from Windows badly." There's a lot of not-invented-hereness to the interface in general, too.
On the plus side, it works quite well, and integrates reasonably well with iPhoto. It makes it a lot easier to justify shooting in RAW because I don't have to worry as much about filling my card up.
It would be nice if the Android app would let you configure the card's WiFi settings, but as long as you have a computer with USB port available you can generally get it set up. There's also opensource Linux tools for configuring the WiFi, and apparently you can also use it to change its upload behavior to go to your own server as well, if you don't want to rely on Eye-Fi's servers for whatever reason.
I do wish the Eye-Fi software itself had RAW support, though; it's annoying to just see the blank 'unknown image format' placeholder icon instead of a thumbnail of the actual image in question. Since I got the card primarily for dealing with RAW it's annoying that they don't even try to support it directly. (I realize that RAW is a gigantic quagmire, but there are some very good open-source libraries for dealing with RAW from basically every vendor, and MacOS also has excellent camera RAW support built-in.)
All in all I'm happy with it, even with its glitchy rough edges. Of course, $50 is a very good price for the 8GB model, so if you're in the market for something like this, it'd be silly to pass it up. At this price I'd get another one but I'm in the process of moving and don't have a reliable shipping address just yet. Sucks to be me.