Last year Larry Hurtado paid tribute to the man and his work in this article: "Martin Hengel’s Impact on English-Speaking Scholarship," ExpT 120.2 (2008): 70-76. (HT: Michael Bird.) Hurtado begins by observing that "In the final decades of the twentieth century, Martin Hengel was probably the most prominent and influential German NT scholar in Englishspeaking circles."
Hurtado specifies two areas of particular note:
First, Hengel has set a high standard of thoroughness of research that continues to instruct and inspire. Second, his frank acknowledgement of his Christian stance and theological concerns is commendable, both in its honesty and in his demonstration (contrary to the anxieties of some) such a commitment can actually inspire dedicated and critical historical analysis that wins the praise of scholars of various faith-stances. Third, over and against both anti-critical conservatism of a creedalistic or fundamentalistic nature, and over and against the now-fashionable disdain of the validity of critical historical investigation in some so-called ‘post-modernist’ circles, and also over and against the tendency by some other NT scholars to play off critical historical study and hermeneutical concerns, Hengel’s body of work stands as a monumental refutation and inspiration. (p. 75)Here is an interview that John Dickson (Centre for Public Christianity) conducted with Professor Hengel in 2008:
Update: See John Dickson's tribute at Zondervan's Koinonia blog.