I firmly believe that The Mentalist is playing fair. By that I mean that Bruno Heller specifically, and all the producers of The Mentalist, have had a firm idea of where the Red John storyline was going right from the start of the show. I'm not suggesting that some 'master list' of the series' plot-intensive episodes exists, or that the producers had any idea what the specific clues or twists in the Red John storyline were going to be more than a few episodes in advance. I do, however, believe that those producers have known who Red John was all along, and have been careful, even as their plots and characters evolved, never to give any information or clues that contradicted anything already established as fact.
Beyond that, I believe that the mystery as presented by the show has been solvable, and what's more, I believe that I have solved it.