Whether you like Ubi Caritas est vera or Ubi caritas et amor this is a beautiful piece.

In the Liber this comes at the Washing of Feet at Holy Thursday - reminiscent of the Mandatum that disciples of Christ should love one another.

I remember hearing a rather striking version of this on the radio - a polyphonic version that started out rather cacophanous and gradually resolved into this well known chant. Maybe not a version to sing at Mass, but it has bugged me ever since, so please let me know if you know of it.