Q&A is a fairly typical Lumet film in the tradition of Prince of the City, Night in Manhattan, and well, everything else since Serpico (with the notable exception of the brilliant Before the Devil Knows You're Dead). In a lot of ways it seems that Lumet got stuck on this corruption in the force thing and devoted the rest of his career to pounding that theme to death. I'm not denying the importance of his subject, but I've found it very easy to see where his films are going. What's odd is that because I'm such a fan with a broad knowledge of his work, I've found watching his movies for the first time less enjoyable.
One good thing about watching these older films (1990 is not that long ago) is I'm reminded of just how good some of these actors were in their prime. Hutton and especially Nolte deliver excellent performances.
Last, I did have trouble with the ending. I don't think the final note of the film was developed as the film's principal conflict. Hopefully that was vague enough to avoid giving anything away.