I have compared songs that were CD ripped to ALAC and the Mastered for iTunes track bought from the store with my Grado 125i headphones.
As a casual listener with mid or entry level audio equipment, I found little difference between the ALAC and the Mastered for iTunes tracks. These difference, in my opinion, are negligible. I can now enjoy a 'compressed' or 'lossy' recording of a music without sacrificing too much sound quality.
I had my reservations with AAC or MP3 files regarding their sound quality. Now, with Apple's Mastered for iTunes, I may have to consider them as an option when acquiring music.
I still prefer having a physical album and rip it to my iTunes library as ALAC. However, considering the amount of disk space a lossless track occupies in my hard drive, having the Mastered for iTunes track is good option for listeners like me.
Tracks that I have compared:
1. Spark by Fitz & The Tantrums
The bass in the ALAC file is tighter. However, the Mastered for iTunes is not that far in terms of quality. Only noticeable, to me, with repeated plays on a 2-second part in the track.
2. Doin' It Right (feat. Panda Bear) by Daft Punk
Same observation from the previous track. By the 29 sec. mark where the bass will first hit you, lossless version is tighter. But this is only noticeable if you are looking for the differences. But if I were to enjoy and really listen to the music, I wouldn't notice the difference.
3. Glad Rag Doll (Alternate Version) by Diana Krall
Diana's voice and her airy singing style in this song is captured by both the ALAC track and Mastered for iTunes track. I found little to no difference between the two.
4. Locked Out of Heaven by Bruno Mars
I just noticed there is a little bit of echo in Bruno's singing at the beginning of the song. The echo itself is not really noticeable in the lossless file and playing it again with the Mastered for iTunes, it's still there but barely noticeable.
5. One Note Samba by Olivia Ong
High frequencies in the ALAC is a little bit smoother. I can't stress it enough that the 'lossy' does not have that much of a difference in sound quality.
In my opinion, if a listener would want great quality music but would like save disk space on your computer, songs that are Mastered for iTunes is definitely something you should consider. If not, then still go for lossless.
*** UPDATE ***
I did a null - test to some of the songs in the list. There are, of course, differences and I would still say that the differences are very small. Only audible when volumes are high. This is very evident when I null-tested Diana Krall's Glad Rag Doll (Alternate Version). The null-test was almost silent. In order for me to hear the differences, I will increase the volume to uncomfortable levels. In my opinion that 'Mastered for iTunes' album is pretty close to the CD. Hence, lossy but with near-lossless quality.
Other tracks that I've null-tested were giving out audible hissing sound even at comfortable volume levels. You may describe those hissing sounds as the 'detail' or 'brightness'. It'll depend on the listener.
Edited by Kiddomike - 6/29/14 at 10:25pm