Kümmel's introduction is a helpful representative of 20th century scholarship. His research is extensive, esp. into German and Continental scholarship. Kümmel is frequently skeptical of coming to conclusions on certain matters and favors critical distance when possible, so a reader interested more in "concrete answers" might do better with Guthrie's or Brown's approach. Kümmel does make some solid resolutions, but only when they are widely accepted by mainstream scholarship. This is a useful research tool, but less so for Christian theology or practice.
This was the foundational text for the introductory course of the Christian Scriptures at Union Theological Seminary taught by Walter Wink. It is one of the most densely packed texts I have ever read, tracing the interpretative history of scholarship regarding the canon back to the origins of such studies and summarizing contemporary opinion. Not the easiest read, it serves very well as a reference.
One of the most honest commentaries on the NT Kummel is insightful and brief. Best introduction to the NT I have come across to date. (More to come on review)