Let's be honest and straight to the point. Fantastic Four is a horrible mess. It's the worst superhero film in recent years. The third act is a mess, character development is nonexistent, character designs were horrendous, the pacing was off and the CGI was bad. There's nothing 'fantastic' about it at all. The origin story for the four heroes was changed entirely in this new reboot. The film has a much darker, serious tone compared to its predecessors. It tries to give a modernized retelling of the superhero team's origins with some unique casting decisions but it failed miserably. The film never bothers to take time to show and develop the relationship or family bond that exists between the four, their reactions to their new-found abilities and how they work together as a team. There's no exploration of how they've been affected physically and emotionally after what's happened to them.
There is little or no interaction between any of the four main characters to let the audience get to know about them. Reed and Ben are portrayed as lifelong best friends but they only have a few scenes together in the film. Johnny and Sue are brothers and sisters (Sue is adopted) but they rarely speak to each other in the film and expressed love to each other like a family. There's no reconciliation or resolution for the character conflicts presented in the film. The alternate dimension where they traveled and gained their powers is a barren wasteland that remains unexplained in the end. Furthermore, there's no development to Victor's drastic transformation into the powerful villain, Dr Doom. It felt like the villain is merely there for the heroes to defeat and save the world. Not to mention, his character design was horrible to the point that you'll wonder why the design gets approved by the filmmakers. Why does there have to be a one-year time jump after the incident happened which caused our main characters to develop superpowers? Why making Ben and Sue mad at Reed for abandoning them when there's no proper resolution at the end? Why does Reed have to escape in the first place? Why does Doom hate Reed? Why Doom thinks that our world isn't worth saving?
Moreover, the visual effects for the third act were so horrible that it looks like it was shot almost entirely with a green screen. This could be the only major superhero film with the worst climactic battle I've ever seen. All these lead to many thinking that something went terribly wrong during production. It is said that 20th Century Fox pulled 3 major action scenes, change the entire third act of the film, hijacked the editing process without the director. The fierce argument between the director and studio caused many unpleasant behaviour from the director on set during production (to the actors and crew). This is not the first time that films turned bad due to studio interference during film production.
Needless to say, this is a terrible reboot for one of Marvel's famous superhero team. It's such a shame considering the fact that the film has great potential, that it might eventually tie in to the X-Men franchise to create a bigger universe. Such a waste of talent, resources and effort to make this film. Please do yourself a favour and save your money instead of watching this rubbish.
Note: There are no mid-credit or after-credit scenes. So don't bother to stay.