I use it a lot. It was frustrating setting it up but works great for me now. I picked up an electric Cecillo here from Fiddlerman and its almost silent. I live in a small apt with my wife, so practicing with my acoustic was difficult was difficult at best, even with a mute.   

If your using an electric you will need to set it up as a guitar, and get your input and output setup right. In both your main preferences AND Garageband.  It seems a little glitchy though so you may need to switch around and back an forth to get sound. Always look at your visual sound monitor at the bottom to see if sound is getting to Garageband before you worry about hearing anything. 

Also, in the track there are 3 little buttons, a mute, headphones, and sound (I guess, they seem a bit redundant, but you need to have the third one on or you will hear nothing. 

For some reason the sound settings seem to change at will (or default settings of other software, I believe its a ghost myself). At any rate, you may need to rest all of these each time you set up itunes. 

One last bit of advice, get good headphones.

Hi Fiddlerman,

Just noticed this was from a few years ago. Ha Ha

Did you get the imovie to work for you? There is a way to do minor stuff like reducing background and fuz, but anything major Garage or Logic would be your best bet. Let me know if I can help.

Chad