The simplistic up/down how "fresh" a movie is the worst part of Rotten Tomatoes. A middle of the road movie that 90% of critics score as 3 stars rates higher than a polarizing film that a 75% of critics find brilliant is the biggest problem with Rotten Tomatoes. It doesn't reward risk taking at all.
And, frankly, what a critic is saying is more important than how many stars a critic gives a film. It's been this way since the star rating first came into effect and the "thumbs up" shorthand of Siskel and Ebert. Context is important.
That said, it's good at giving a good snapshot of a broad section of opinions and has handy links for more in depth reviews. Both are handy to have when used as a tool not a crutch for decision making.
