Check out this website:
http://www.hostelworld.com/
If you search for hostels in a particular city, each hostel listed is given an overall percentage score, awarded by people who've stayed there in the past 6 months.
If you click to see the details of a particular hostel, you'll see more detailed percentages, covering 'character', 'security', 'location', 'staff', 'fun' and 'cleanliness'.
If you then click the 'ratings' tab, you can read the individual comments of everyone who has reported on the hostel in the last 6 months.
Chris