An absolute must to consult when organising any trip. Look up an area you plan to visit, and check out some of the reviews posted on the main hotel/guesthouses, or just type in the name of the guesthouse you have already selected to read what previous visitors said. But you need to use some judgement: some reviews can be posted by "friends" (you will notice that the reviewer has never posted a review elsewhere) or "enemies" (some competitors use the site to malign a place they feel is taking away business from them). Also, some negatives for one person (ie "no night life" or "not child friendly") can be a plus for another person ("calm and peaceful"). Note also that this website belongs to Expedia.com (as does venere), so places working with these discount sites will be listed prominently.
With these caveats in mind, an absolutely indispensible tool for any traveller.