The percentages aren’t a grade of quality, but instead give a rating based on whether critics did or didn’t like the film as a whole. The thing is, a film with a high ‘Fresh’ rating could consist of reviews claiming it to be a cinematic classic, but it could just as easily be that critics agreed it was only “alright”. For that reason, it’s an inaccurate way to judge the quality of a film, since reviews are reduced to simply ‘good’ or ‘bad’. It would be illogical to compare film scores; Toy Story has 100% on Rotten Tomatoes, as does Citizen Kane. Does that make Toy Story as good as Citizen Kane? Well, it is, but those scores just mean that everyone likes those films and nothing more (also, Toy Story and Citizen Kane aren’t exactly comparable!).
Other examples include the recent Marvel films, such as Guardians of the Galaxy, Captain America: The Winter Solider and The Avengers, which all have consistently good reviews averaging at around 90%. That’s not because they’re deemed to be masterpieces like certain films with the same score, but because they’re good for what they are: entertaining blockbusters, with everything you could want from the superhero genre.
Ultimately, the chances are that you’ll like a film more if it has a higher rating, although that’s certainly not always the case. The Tree of Life received high critical acclaim (with 84%), but I thought it was a load of pretentious nonsense. I’m not at all opposed to that kind of introspective, meditative film (I love Tarkovsky’s Stalker, think 2001 is great and enjoyed Take Shelter), but for me it was two and a half hours of out of focus low angle shots without a lot to say. The whole ‘birth of the universe’ part was cool, then it all went downhill (that sounds like a Douglas Adams quote). The 60% user review score on the site (which is always interesting as a comparison) shows that I’m not exactly in a small minority in terms of the general audience's opinion, but just because a film has a good score doesn’t necessarily mean it’s automatically ‘good’.
Subjectivity is the most important thing with any kind of medium; every review is written by an individual with different life experiences and opinions as to what makes something successful, and a film can resonate differently depending on the person. Indeed, it's also worth noting the film's targeted audience and those reviewing it. Roger Ebert gave Fight Club, arguably a modern classic, a bad review, but you could say he was already a part of the system the film was criticising, and that he missed the point when dismissing it as simply “macho porn”. The Inbetweeners is targeted at a very particular demographic, making reviews by middle-aged Telegraph writers largely inconsequential, as well as the fact that those who like the series and the films, including me, already know what we’re in for, so the score is a bit redundant anyway.
In the same way that I might disagree with a film’s high rating, there are others which I believe are underrated. I loved Mr Nobody (67%), and think Terry Gilliam’s The Zero Theorem is one of the best releases this year, despite it only having 53% critically and a 46% audience rating. This is the problem with divisive films, where there’s a clear distinction between those who thought very positively about a film and those who didn’t, in that 50% approval is considered Rotten. True, most positive reviews for a Rotten film aren't likely to be glowing, but there are certain examples of polarising reviews. So to rule out anything below, say, 60% or 70% excludes a vast amount of films which you might actually like, or at least get something out of. Even the objectively awful films can be worth a watch, precisely because they’re objectively awful – looking at you, The Room… Some films are perhaps not very good as a whole, but have scenes in them which make them worth seeing. A good example which springs to mind is the mediocre (at best) Knowing (33%), starring the brilliant/awful/insert-adjective-of-choice-here Nicolas Cage. It’s all pretty forgettable, apart from one incredible and terrifying scene in which a plane falls out of the air, shot in (what appears to be) a single take which would make Alfonso Cuarón proud.
Rotten Tomatoes is great for a general overview of how many people liked the film, but without looking at the reviews in full, it’s nothing more than that. It’s obvious and doesn’t really need saying, but the only reviewer you can trust is yourself; a film is almost never entirely good or bad, and even then it’s largely subjective.