Surprisingly good. Lang may of course come across to some as a bit of an obsessive gadfly, but thanks to his 'files' this book does science and society a sorely-needed service- it exposes responsible parties' tribalism, evasions and cognitive dissonance, all in their own words! This approach of reproducing correspondences, witness testimony and extracts from reports makes for lengthy accounts, but it does a fair job of repelling the common (because commonly accepted) but too often unjustified 'out of context' or 'misinterpreted' defences.
The Baltimore case was a particularly interesting read. It exposes what I think are real dangers of charismatic leaders in science acting against the best interests of the profession and its careful standards, sometimes by rallying it against overhyped 'outside threats', to absolve themselves of responsibility or to cover-up misconduct that they view as mundane or not worth the trouble. Lang is a great critic of facile alternatives (see for instance the chapter on the Ladd-Lipset case) and it cuts through the whimpering evasions of the case decisively.
Lang's most problematic view is that he presents here a stance disputing HIV as the cause of AIDS. Or perhaps I should say his stance is that the hypothesis 'HIV causes AIDS' too soon became dogma without the necessary support. From a skim of the chapters on this view, Lang might indeed have seized on some problems of methodology and politicised motives and review, but it's unfortunate that he didn't revise his views in light of further, more rigorous evidence as it mounted.
Despite this qualm, Challenges might do well if it were restructured and reprinted, with much of the support material going into appendices, say.