If you look at their listing for HAIRSPRAY, for example, you can see that the site also excerpts portions of all the reviews from which they determined the grade. It’s a good way to get a sense of what lots of different writers think of a show.
From their explanation of the whys and wherefores:
Critic-O-Meter is an idea borne of the blogosphere. Critic and editor Rob Weinert-Kendt and myself (director and writer Isaac Butler) were having matzoh ball soup at the Polish Tea Room when Rob mentioned that he had read on a blog that someone said “You know, they really should have a Rotten Tomatoes for theatre reviews”. In true slowest-moving-art-form fashion, we then puttered about talking about it for a few months before finally building the site you see before you today.