I too often find myself in the uncomfortable position of having to take the biographer's view of things when reading biographies. Of course, it is possible to see that some are a little too enamoured by the people they write about, but other than that it can be difficult to judge. I therefore tend to end up judging it on stylistic criteria. And I'll check up on the facts if they seem outrageous enough, of course.

Too much detail can kill a book. Some writers seem to think that just because they found something out it must be presented in the book. This is more forgiveable in a biography than in fiction, of course, but still...