By purchasing a “Mastered for iTunes” release, you are buying a product that has been mastered and quality-tested for publication in iTunes. If you could compare a regular iTunes (from CD master) release and an MFiT release, listening on a high fidelity, full range sound system, you would notice improved clarity of transients and dynamics of the music with this new iTunes mastering format. The intent with MFiT, is if you had access to a 24-bit uncompressed master and compared it to the MFiT version, they should sound virtually identical to the majority of listeners.
Anyone listening on a high-quality playback system, in an environment that allows them to hear all of the details of the music, will be rewarded with a wider dynamic range (a.k.a. the difference between the very loudest and the very quietest parts of the music), a slightly lower noise floor, and less distortion (which translates into more sonic detail in the music).