Have you ever started watching a movie then realized about 5 minutes into it that it sucked and that you'd be wasting time by continuing to watch it? At that point you have 2 choices - if you're at the movies, you can either walk out or suck it up. When my father-in-law and I went to see Predator vs Alien 2 a couple of years ago, I realized about 5 minutes into it that the next hour and a half was going to be extremely painful. I hated the movie and almost fell asleep several times. But that's if you're at the movies. If on the other hand you rented a film from Blockbuster and didn't want your $4.23 to be wasted, you may have forced yourself to sit through it. Of course you also could have decided that no amount of money that you'd be losing would make you watch it and that you would actually pay to not have to watch the movie. For me its aggravating to see a preview or read about a movie, be interested in it, go to the theater to see it, rent it, get it from Netflix, whatever, and realize its crap. Thankfully with Netflix (or Blockbuster.com) if you don't watch it, you're not out any money.
So after wasting money at the theater on crappy movies or wasting my time waiting for movies to come in the mail, starting the movies, then stopping them, I thought that reading film critic reviews of movies might help me make a better use of my time. But as I read reviews, I realized that critics have different tastes - 5 film critics might give 5 different reviews - and I realized that I couldn't really trust a critic. Its not that critics are completely unreliable, but I wanted to hear from more than one critic.
A couple of years age I found a site called Rotten Tomatoes which is all about movies. My favorite feature of it is the ratings - RT compiles reviews from loads of film critics then gives an overall rating. So if you go to the site and search for Twilight (a movie I would like to see), you'll find it has a 49% approval rating. So the critics are split down the middle on that one. I'll probably still watch it anyway since its a Vampire flick. Watchmen got a 64% rating, so I'm debating on whether or not to see it in the cinema or wait until it hits DVD.
Anyways, I prefer not to waste my time (and money) if I can help it. Visiting Rotten Tomatoes for a few minutes helps me decide if I want to invest money as well as 2 hours of my life that I can never get back in a movie.