The Gospel of Judas has genuine historical value—as one of several bits of evidence showing the diversity of early Christianity, like the writings of such figures as Irenaeus, the bishop of Lyons in about 180 A.D. The text's depiction of Judas as the disciple to whom Jesus gave unique mystical revelations is not itself really unique. It somewhat resembles the portrayal of Thomas in the Gospel of Thomas. Nor is there evidence that the Gospel of Judas ever enjoyed much popularity as an alternative to the canon of the New Testament or was considered for inclusion in that canon. This text reflects a profoundly elitist viewpoint, claiming a specially conveyed revelation of religious truths withheld from ordinary Christians and their leaders.
I am looking forward to reading a traditional papyrological description of the manuscript, as I wonder what else this text was bound with. Apparently it also contains a variant of the Epistle to Philip and the Revelation of Jacob, the rest being unreadable, but I have been unable to track down Kasser's original comments on this point.
(Update 4/14: It turns out there is good reason that news on these manuscripts has been so sketchy. No one seems to know who has what and how or when they got it. See above post for more on this.)