More important than the controversy surrounding him is John Cassian’s legacy. This legacy can be seen in East and West in the history Christian spirituality and monasticism.
In The Institutes, John Cassian presented his adaptation of Evagrius Ponticus’ teaching of the eight thoughts most to be avoided. Cassian’s eight vices — Gluttony, Fornication, Avarice, Anger, Sadness, Acedia, Vainglory, Pride — were adapted by St. Gregory the Great (540-604) into a list of Seven Deadly Sins. He combined vainglory with pride since the two vices are so similar. The Seven Deadlies have influenced thought right up to this day; the only person I can think of whom you might be interested in reading on this topic is St. John of the Cross, The Dark Night of the Soul.
In the Rule of St. Benedict, Cassian’s Conferences are recommended reading for the monks. The result of this is that many aspects of Cassian’s spirituality run throughout the spiritual writings of the Benedictines (and thence the Cistercians, Carthusians, and so forth). As well, however, St. Benedict encourages his monks to begin their prayers, “O God make speed to save me, O Lord make haste to help me.” This is a bit of advice from Cassian’s 10th Conference, where he waxes eloquent on the usefulness of that phrase from the Psalms. To this day, if you go to a Prayer Book service in the Anglican Church, that is right near the beginning of Morning or Evening Prayer (it usually follows, “O Lord open thou our lips, / and our mouth shall show forth thy praise).
This is probably the best we can do for the obvious, visible legacy of Cassian in the West. The controversy and the centuries have not dealt with him over here kindly. Nonetheless, his influence no doubt runs through the whole current of monastic spirituality, which is itself the spring from which much of the rest of Christian spirituality draws.
In the East, Cassian’s teaching on Grace & Freewill is understood by some to be the orthodox Orthodox position. I’m not sure that they are as obsessed as we are about having a perfect definition of this doctrine, however. Nevertheless, he has the notable distinction of being the only Latin writer who is included in the Philokalia, the Eastern Orthodox collection of teachings from the 2nd through 15th centuries. These teachings centre on prayer and are the core of most Orthodox spirituality. This is where the bulk of his influence in the East is found.
Since Cassian holds a position within the central texts and traditions of Christian spirituality, both East and West, I believe that we should all read him — Roman Catholic, Eastern Orthodox, Protestant. We should seek to understand his teachings and draw towards purity of heart and the vision of the divine.