I tried a $9 instrument to USB adaptor, and a $50 Behringer unit. $41 extra dollars didn't help my experience.
I think I've mentioned that I am a terrible guitar player. In a misguided, and luckily brief, attempt to record a song I'd written, I decided to find a direct to Apple's Garageband adaptor for my Fender Jazzmaster. I decided to try a very cheap, no name plug, and a very nice tabletop box.
The cheap USB dongle works! I plugged it into OS X and it just magically worked. Any adjustments I wanted to make to the guitar's sound had to be managed inside of Garageband, but I was able to record myself playing. Amazingly, I sound worse recorded than when I am listening to myself live. If all you want to do is record some music out of a guitar with a 1/4" cord, this will get the job done.
The Behringer UM2 is a sexy unit that does a bit more than the USB dongle. Gain control, output level, and offering a 2nd input, for a mic or 2nd instrument are pretty much it. You can get 5 of the USB dongles for the same price. With my absolute zero home recording experience, and miserable ability as a guitarist, the Behringer is overkill.
Behringer UM2 Audio Interface via Amazon