I just listened to this:
So the introitus and kyrie.
It is beautiful music and I am glad to be aquainted with it. I can certainly hear the influence of this work on Mozart.
However - it is not, for me - on the same level on artistic invention. I would say it is closer to the level of Pergolesi Stabat Mater - which I do like a lot and it is no insult for me to say that. Mozart is known for taking pebbles and coverting them into diamonds and that is the case with Haydn's fine mass.
There is a moment toward the end of the Sequenz that stood out to me even before I was very familiar with the piece, and I now recognize it as one of the most haunting and beautiful moments in the whole work.
It is the words "Judicandus homo reus" in the Lacrimosa. The music he sets those words to....... my god.
I'm still obsessed with this piece. I've now listened to the entire thing about 37 times since I purchased it in July.