This set is cheap, and it is an easy way to collect almost all the masses. If you like church music and Mozart, it is hard to see that you will go wrong by buying this set. On the other side, I am not totally convinced about the music.
The music is good, and the recording is crispy, no doubt about that. But I find everything too soft, especially the requiem. My favourite requiem is Karajan’s recording with the Berliner Philharmonic, which is raw and brutal. The requiem on this box-set reaches Karajan only to his ankles. But if you want a requiem that suites a warm bath, this one is probably better.
I miss a libretto. When they sell you a huge box, they can add a libretto without to much of a hassle. Fortunately, you might have the libretto already (it is not an opera :), or they are readily available on the net.
Conclusion: This set is good, but consider adding a couple of CDs; at least another requiem and maybe “Davidde penitente” to compare it with “Mass in C minor”.