I have a registered copy of Soundjam and am a great fan of it's many abilities. It performs smoothly and the sound is excellent. The interface (as with all mac things) is easy to use and very expandable in the sense of downlading new skins and visual plug-ins. The CD converter is a feature I rate as top knotch in that it is efficient and smooth running. I think Soundjam is the best program out there currently for the mac when it come to playing mp3s. My advice is to download the free version and try it out. You can even convert some of your songs to mp3 format to see how you like it. Many of my friends have stuck with the free version because of its abilities. As for quicktime, I didn't care for it. Nor did I like audion or grayamp. It all depends on what you are looking for however.
Good luck!