I guess it depends on what you mean by "reliable." It looks meticulously researched, and the CAP folks argue that their approach is objective. This may actually be true. What I wonder is whether the standards they apply to movies they watch are the same as the standards you want to apply to your life.
When I decide whether I will go see a movie, I do not concern myself with whether I will see something that violates any particular standard of content; my decision is based on whether I think I will find the movie to be a fulfilling experience, either artistically or for some other reason. So the CAP database would not be useful to me even if my standards and theirs were the same.
I do, however, have children, and I am responsible for ensuring that they are not exposed to material I think they're not ready to handle. Since my standards and those of the people behind CAP are not the same, I prefer to rely on the user-contributed "parent's guide" feature at imdb.com, which lists scenes in the movie which might be of concern to parents in the categories of Sex and Nudity, Violence and Gore, Profanity, Alcohol/Drugs/Smoking, and Frightening/Intense Scenes. another IMDB advantage, or course, is that it gives me access to reviews and other commentary that can tell me that a movie, aside from being offensive and inappropriate for children, is also not worth watching simply because it's a bad movie.
Finally, it looks like CAP is behind the times. I couldn't find an entry for Watchmen at CAP, though there is a guide for parents
about the movie at IMDB (and it is very, very, very long).