I would still pick up a book if i am attracted by the blurb, even with low reviews. Also, a lot depends on what the reviews actually say. What some people may feel as bad, some people make actually like.
To me it depends. Most of the time it is about the subject or theme, although I also look at reviews to see if it has substance from those who have read them. The author matters if I know he/she has relevant information, detail and credibility to add to the genre. In other cases, it is more a matter of context, to see what was happening at certain times and places.