The initial seasons were generally well-written, particularly regarding how realistic characters acted and spoke; it was refreshing to see so much common sense applied. However, around the middle of season four, the writing quality became erratic, with noticeable drops in believability across those same areas. A new character introduced felt incredibly irritating, and even attempts to soften the portrayal of their psychic abilities didn’t help. There's also a scene involving a bank robbery where three security guards are present in an office for two minutes before anyone bothers to activate an alarm – it just doesn’t make sense. Furthermore, gunfire sounded unrealistic, like firecrackers. It seemed as though everything started declining, almost as if the production lacked funds to maintain even basic television standards. It feels like the show is simply trying to capitalize on its earlier success and I'm very close to abandoning it altogether—it brings to mind what happened with Navy CIS after Gibbs left. While not every episode was bad, some are easily skippable, which wasn’t a problem in the beginning. It’s common to see shows run out of story ideas, forcing writers to invent new plots and cutting corners on things like plot holes and inconsistencies. Because of this decline, my rating is influenced; the first season would have earned an easy 90% recommendation, but I'm not sure anything past season four warrants more than a 60%.

Source on TMDB