I know I'm a little late on this, but I have to note Professor Hengel's passing because of his indirect influence on me via my advisor, Don Hagner. I never met Professor Hengel, but Dr. Hagner knew him well and spoke glowingly of him, particularly his carrying on of the Schlatter tradition of NT scholarship that wedded the historical-critical method to a believing stance toward the NT. According to Dr. Hagner, Hengel had a picture of Schlatter hanging in his study.
Here's a round up of more obits and remembrances.
David Neff has a good piece in the CT blog, The Champion Who Debunked Bultmann.
Also, Justin Taylor has an excellent quote by Larry Hurtado (summarizing the same sentiment that Dr. Hagner has expressed) and a link to an interview Hengel did last year for CPX.
Hengel was part of the evangelical revival at Tübingen, along with Peter Stuhlmacher and Otfried Hofius. Dr. Hagner was a visiting scholar there sometime in the 80s I believe, where he got to know Stuhlmacher and Hengel personally and became good friends with them. He is deeply influenced by their approach to NT scholarship.
Dr. Hagner tells a story of one of his visits to Germany for a NT conference at which Hengel was a speaker. Afterward, on the train ride home he overheard some German scholars of a more radical critical bent complaining of the "fundamentalism" that had lamentably taken over the NT faculty of the once-great Eberhard Karls Universität Tübingen (remember, modern historical-critical NT scholarship basically starts with F. C. Baur at Tübingen in the 19th century).
Another Hagner-told Hengel story. Hengel was a great proponent of ironically using the historical method against those who purported to use it to attack Christianity or the reliability of the NT.So once, when asked about some recent sensational claim (I don't remember which one, maybe it was something from the Jesus Seminar), he raised his right hand repeatedly and brought it down in a swinging motion, "Es ist nicht wissenschaftlich! Es ist nicht wissenschaftlich!" spoken in harsh teutonic tones with the word nicht emphasized.
My other mentor at Fuller, Dr. Seyoon Kim, also has great respect for Hengel and Stuhlmacher, not only in terms of method, but in their approach to Paul as well. Hengel's book The Pre-Christian Paul (co-authored with Roland Deines) is brief but very helpful.
When the Fuller NT faculty was looking to hire an experienced NT professor, Roland Deines was the candidate that Dr. Hagner wanted most, because it would have promoted his dream of Fuller becoming a kind of American Tübingen at least in the field of NT studies. I didn't work out and Joel Green was hired instead. Joel Green is a competent evangelical NT scholar doing good work in the NT use of the OT, but he doesn't seem as interested in carrying on the kind of detailed work with the sources of Second Temple Judaism and early Christianity that Hengel was renowned for (and Deines as his protege). Another reason the choice of Green over Deines is disappointing is that Green is not clear and firm on the doctrine of substitionary atonement as Hengel was.
I think Hengel's legacy will ultimately be his example as an evangelical Christian who was a scholar of the heavyweight category. He took the time to master the primary sources in the original languages. If you want to be inspired to make that sort of linguistic effort, read Hengel's 2001 piece in Christianity Today exhorting today's young evangelical scholars to master the sources of ancient Judaism, Greco-Roman literature, and the church fathers.
Well, a great scholar has run the race and finished the course and gone to his reward. He will be missed by many.