Lately I've been experimenting with iTunes Plus settings. It is excellent quality but I feel my heart will always be with MP3. I find my reasons to use MP3 is because universal comptability and well I'm a Windows user and won't see myself switching anytime soon. So what codec do you use?
I use AAC as it's a slightly higher quality codec. The advantage of MP3, from a Mac point of view, is that song lyrics are searchable through Spotlight, but not with AAC. Also, not to mention the wider spread usage of the MP3 codec in third-party equipment. I comes down to what you want to use. I do all my songs at 160 Kbps. That's a fair balance between quality and size.