iTunes built-in conversion would be fine, other than you then have two copies of each song, the original and converted. This program is nice in that it removes the original and automatically updates iTunes to point to the new file. Of course, converting from one lossy format to another (MP3 to AAC) isn't ideal for great fidelity, but if you have a large collection of audio books on your hard drive, where fidelity isn't as critical, this is a great way to crunch them down to 50 percent of iTunes default settings.