So, it was quite the breath of fresh air to see comments made by C.K. Barrett in the preface of his first edition (1955) classic The Gospel According to St. John (SPCK) regarding Bultmann's monumental The Gospel of John: A Commentary.
Dr. Rudolf Bultmann's commentary is beyond question one of the greatest achievements of biblical scholarship in the present generation. Any one who compares my book with his will see that I have ventured to disagree with him on a number of important issues, but I have never done so without hesitation and doubt. The value of his commentary is almost completely independent of the validity of his literary hypotheses (vii; italics mine).
These comments remind of the one's recently made by J. Ramsey Michaels in his commentary on John (NICNT):
So if these two great commentators, Barrett and Michaels, find Bultmann's work invaluable, despite the inherent difficulties with his source theories, etc., shouldn't we take a cue and read Bultmann with admiration and appreciation as they have?
To my surprise I found Rudolf Bultmann’s commentary the most useful of all, a work widely admired for all the wrong reasons. Bultmann’s theories of source, redaction, and displacement has not survived and should not, yet his eye for detail is unsurpassed and his close reading of the text as it stands — even when he discards it — perceptive and illuminating. It is only a slight oversimplification to say that Bultmann interprets the Gospel correctly (more or less), finds it unacceptable, and then re-writes it. His greatness lies in the first of those three things, not the second or the third (Preface; xi.).