Here's my problem with rotten tomatoes. Someone correct me if I'm wrong, but it appears to be a composite of binary reviews. So they set a minimum for what would qualify as "fresh" (1) vs. "rotten" (0) for each individual review. When in fact, most reviews are not so cut and dry. An example would be if say, the boston globe gives a movie the equivalent of a 6 or 7 out of 10, RT may qualify those scores as being a "fresh" review - so it is counted as a 1/1. While a review that's on the high side of bad, maybe a 5/10 would be counted as "rotten" and counted as a 0 in the final percentage rollup. So I notice this tends to skew movies that are ok-but-not-great one way or the other.
That said, if you're only looking to watch 90+%, grade A movies, it works great. Generally I find their very high (85+) and very low (15-) ratings are pretty accurate.