Honestly, I am not sure where all of the criticism is coming from. It's a thoroughly enjoyable thriller, with constant plot twists riddled through it. It's very similar to both Angels and Demons, and The Davinci Code, so if you considered those mediocre or dull, then expect nothing different here. Many of the critics, it would appear, hated the clue-hunting, historical side of the film. So those critics were, once more, disappointed with the clue-hunting and historical aspects of the film. However, if you saw the first two films and loved them like I do, then you won't be disappointed, as personally, I believe that this actually brings a lot more to the table.
Either way, if you liked the first two, but are concerned about what the critics have to say regarding Inferno, take a look at the reviews for the last two films before you write this film off. The reviews are overly harsh, and many people find the historical fiction and puzzle-solving dull, which if you do, would make the film unbearable. If you don't hate those aspects, then the film is the cherry on top for this trilogy.
Either way, if you liked the first two, but are concerned about what the critics have to say regarding Inferno, take a look at the reviews for the last two films before you write this film off. The reviews are overly harsh, and many people find the historical fiction and puzzle-solving dull, which if you do, would make the film unbearable. If you don't hate those aspects, then the film is the cherry on top for this trilogy.