I own this set and I think it's a superb introduction to sacred music. Imho, it's the best introduction one could get (other options include some sets from Brilliant classics). It's 29 cd's of music, as the 30th cd is a pdf with the sung texts.
As far as the quality goes, it ranges from very good to splendid. I think that Harmonia Mundi is the best label for sacred music, along with Hyperion.
As for the selection, I think there are 3 minuses:
1-) The cd on Lamentations is very incomplete; I like very much Couperin's 3 Leçons and Krenek and Charpentier whole cycle but they were not included in their entirety. I had to buy cd's separately (*sigh* those pieces are so beautiful that I had to toss some money for the extra cd's)
2-) The same is true for the Rachmaninov's Vigil in the cd on Ortodox Chant. (since I am not a huge fan of Ortodox Chant, that wasn't a huge problem)
3-) Bernstein's Mass should never be included, period. It's not a problem of the rendition on disc...the thing is that it's not sacred music. I really dislike this "mass".
There are so many good things that I may forget some. But here it goes:
1-) Cd 1 featuring Ensemble Organum's plainchant (Ambrosian, Old-Roman, Beneventan and Mozarabe)
2-) The Gregorian Requiem Mass on cd 2
3-) The motets on cd 4 and the masses on cds 5 and 6 are magnificent sung and so is cd 7 on Baroque motets which I didn't know before.
4-) The music on cd 8 is so good that it led me to spend more money, not necessarily from those featured on this disc. For Krenek's Lamentatio, which I consider one of the greatest choral music from 20th century along with Pärt's Passio, I am not aware of any other rendition but the RIA's rendition featured on this collection is marvelous; as for Lassus's Lamentation, I think that Pro Cantione Antiqua has a slightly edge. For Couperin's 3 Leçons there are plenty of good options out there: the one here in this disc is good enough - there is a disc from Harmonia Mundi with the 3 Leçons; another one I recommend also features 2 countertenors - Daniel Taylor and Robin Blaze with the Theatre of Early Music (Bis); and finally there another great one featuring 2 sopranos - Sophie Daneman and Patricia Petibon - directed by William Christie, with a French pronunciation (Erato). As for Charpentier, unfortunately, it's very hard to find the whole cycle; usually they fill 3 discs and I could not find the 3 of them to buy (only at prices I don't feel like paying. The one recorded in this cd and another by Il Seminario Musicale (Gerard Lesne) are the ones I like the most.
All in all this cd is good but very incomplete; if you happen to like Lamentations as much as I do it's disappointing, specially because those from Charpentier cannot be found complete elsewhere.
5-) Monteverdi's vespers (cd's 9 and 10)
6-) Cds 20 and 21 on the Stabat Mater are awesome. Pergolesi's Stabat Mater is sung by a countertenor and a soprano boy; very unusual but good (it's not perfect, but it's the best I've heard). Vivaldi's Stabat Mater is sung there by Andreas Scholl and Boccherini's Stabat Mater is sung by Agnes Mellon. Both of them are at their best there!
7-) The two discs on Requiem masses and Poulenc's motets and mass. Don't junk Bernstein's cds, because Poulenc's gems are in the same disc as Bernstein's.
You may wanna check the booklet:http://www.harmoniamundi.com/__media/document/3216/Booklet2908304.pdf