Every time I've gone to a movie without reading a review first, I've regretted it. In the past six years, here are the movies where I decided "I want to see this so bad, I'm not going to ruin it for myself by risking reading reviews that might discourage me from seeing it": "Superman Returns", "Spider-Man 3" and "The Hitchhiker's Guide to the Galaxy". All big disappointments. Since 2007, I haven't gone to a movie without checking the reviews first, and I find that the number of movies I end up disappointed in has decreased significantly. Occasionally there are still movies that let me down despite getting positive reviews (i.e. "X-Men: The Last Stand", "Drag Me to Hell", "Scott Pilgrim vs. the World"), but those are increasingly rare.
It's fairly easy to read a review before a movie without spoiling it for yourself. I think the best way is to just skim through it, skipping the parts where the reviewer talks about plot-related stuff, so you're not spoiled if you decide to see the movie after reading. Being familiar with the reviewers, I have a pretty good idea of when they're going to go into plot stuff. Generally the opening and closing paragraphs are where you get the analysis/reflections that are going to make a convincing or not-so-convincing argument about the movie being good or bad.
To determine how credible or relevant to you a reviewer's opinion is, I suggest considering what you thought of movies they praised or put down in the past. For example, Devin Faraci was practically orgasmic over "Hot Fuzz", "Inception", and "Scott Pilgrim vs. the World", all of which I thought were okay, but very flawed, and far from the monumentally perfect masterpieces of film he made them out to be. I often agree with his criticism or complaints about many movies, but I keep in mind his benchmarks for high quality every time he praises something to high heaven. That's why I'm not tempted to rush out and see something as soon as he goes apeshit over it.