An interesting, if not highly technical, piece on the effects of "Mastered for iTunes" on Ars Technica:

"After our original report on the Mastered for iTunes program, some readers were skeptical that anything could be done to make a compressed AAC file sound comparable to uncompressed, 16-bit 44.1kHz CD standard audio. Others believed users should have access to the original 24-bit 96kHz files created in the studio for the best sound. Finally, some readers suggested that few people can actually tell the difference between iTunes Plus tracks and CD audio, so why bother making any effort to improve iTunes quality?...

"Shepard applauded Apple's technical guidelines, which encourage mastering engineers to use less dynamic range compression, to refrain from pushing audio levels to the absolute limit, and to submit 24/96 files for direct conversion to 16/44.1 compressed iTunes Plus tracks. However, he doubted that submitting such high quality files would result in much difference in final sound quality. Shepard's conclusions led CE Pro to claim that Mastered for iTunes is nothing more than "marketing hype."

"So, we set out to delve deeper into the technical aspects of Mastered for iTunes. We also attempted to do some of our own testing to see if there was any difference—good or bad—to be had from following the example of Masterdisk.

"We enlisted Chicago Mastering Service engineers Jason Ward and Bob Weston to help us out, both of whom were somewhat skeptical that any knob tweaking could result in a better iTunes experience. We came away from the process learning that it absolutely is possible to improve the quality of compressed iTunes Plus tracks with a little bit of work, that Apple's improved compression process does result in a better sound, and that 24/96 files aren't a good format for consumers."

http://arstechnica.com/apple/news/2012/04/does-mastered-for-itunes-matter-to-music-ars-puts-it-to-the-test.ars

Over 140 comments made in the last 16 hours since the article was first posted--ranging from know-plenty to know-nothings (but think they know-it-all).

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