Parker, I bought my wife the Eye-Fi for our anniversary and it's been a hit. I recommend it for:
* The spouse who has most everything already, and is a photo enthusiast
* The spouse who doesn't transfer photos to the computer until six months and four outings have passed (and for the computer / data maintainer spouse who finds this stressful...
)
* Anyone who wants a geo-tagging upgrade to their existing camera for much, much less than the cost of a new camera
* The gadget enthusiast
Yes, it's as much as 4x the price of a cheap 4GB card from NewEgg, but ... well... this is a forum where people spend $1,000s or $10,000s on HT toys. What's $60 extra for an SD card? And for regular use, 4GB is more than enough for my wife.
She has a Canon SX-10is -- it's the '08 or '09 top superzoom from Canon. The wifi-based geo-tagging has worked well so far, though we've not traveled much with it yet. I like that when I'm shooting product photos for eBay, it auto-uploads to her Mac as I'm shooting it. That means she can start fixing my white balance problems before I'm finished. She likes the geo-tagging feature. And I like that photos are getting off the camera and backed-up sooner 
The one problem is that the camera can shut itself off in the middle of a large photo transfer. So, she goes out, shoots 400 photos, and comes back in. Turns the camera on, and the EyeFit starts transfering photos. It's 15 minutes of transfer, but the camera has a 5 minute auto-shutoff timer. A minor inconvenience and only hits us with big shots, like vacation.