Jacob Jervell, whose very different 1998 German commentary on Acts replaced Haenchen's in the same series, described his predecessor's contribution as "the most comprehensive and important work of the critical German post-World War II research on Acts."*
(As an aside, Martin Rese describes how "From the end of the sixties until his death in 1975 Ernst Haenchen took me as one of his discussion-partners. He called me up nearly every day for at least an hour, and we talked about the problems of interpreting Luke-Acts and the gospel of John."** What would that have been like, I wonder?)
(As a second aside, I note with dismay that, according to this German Wikipedia entry, Haenchen was a member of the Nazi party during WWII: That decision may have secured him a promotion during the War, but it cost him his official position, at least, after it was over. Update: See Joseph Tyson's longer discussion of Haenchen here. Tyson does not mention Haenchen's membership in the National Socialist party.)
For my part, I share W. Ward Gasque's judgement that the commentary "is in every way a magnificently impressive piece of scholarship -- a treasury of bibliographical, philological, and exegetical detail. ... Even when one does not agree with the conclusions of the author ... he must confess that Haenchen has made him look at the text and the problem raised by it from every possible angle" (A History of the Interpretation of the Acts of the Apostles [Hendrickson, 1989], 243).
To be sure, Gasque immediately went on to add:
"When all is said and done, however, it must be noted with a sense of disappointment that it is probable that Haenchen's great commentary will be regarded by future generations of scholars more as a historical phenomenon belonging to one era of the history of exegesis than as a lasting contribution to New Testament research. ... [W]hen the storm has subsided and New Testament critics are in a position to look back over the past two or three decades of research from the perspective of history they will, I think, be able to see that the commentary of Haenchen is as tendentious and ultimately as unhistorical as he thinks the author of the Book of Acts was." (Gasque, History, 244).
For better or worse, Gasque's prediction has not come true: Haenchen's commentary remains a classic, and the recent excellent critical commentary by Richard Pervo in the Hermeneia series stands very much in the same tradition. (See this post for my initial reflections on Pervo's commentary.)
*Jacob Jervell as translated and quoted in Martin Rese, "The Jews in Luke-Acts: Some Second Thoughts," in The Unity of Luke-Acts (J. Verheyden, ed.; Leuven: Peeters, 1999), 185-6.
**Martin Rese, "The Jews in Luke-Acts," 185.