This is a beautiful volume and a must read for anyone interested in early Christian doctrine and practice. Irenaeus was a student of Polycarp who was a student of the Apostle John. With the apostle's preaching still fresh in his ears, this early Bishop fought the deadly heresies of Gnosticism with intelligence, faith, and perseverance. Here you can learn the early Church's beautiful doctrine of the Eucharist; the relationship between scripture, church authority, and faith, and even some interesting anecdotes about the apostles. You won't be disappointed!
I should also mention that Ex Fontibus has charitably left many of the humorous footnotes from the original edition intact.
UPDATE: (2/29/2012) This is all 5 books of Against Heresies in one volume.