I am Legend. Things start getting really cool... and then it ends.
This times 20,000,000,000!!!
I read the original Richard Matheson story shortly before seeing the film, and boy was it butchered, and I don't mean in the typical hollywood way where things are left out because they wouldn't flow well in the limits of a ~2 hour visualization of the novel, but rather a complete bastardization of the entire concept.
I'll go ahead and assume that everyone has already seen this that wants to, but if not, then minor spoilers ahead, these are the fundamental divergences from the superior source material :
(1)- It's QUITE clear in the novel that what he's dealing with is vampirism.
(2)- The 'vampires' are approximately human in strength, and with varying degrees of coherence. Creepily, some of them talk and taunt the protagonist. The creatures are NOT superhuman CGI zombie retards that flip SUVs and climb 10 story sheer buildings with their fingers.
(3)- He is fooled by a very eloquent 'vampire' that has worn makeup and masquerades as another human, to infiltrate his facility and gather info on him.
(4)- He is eventually captured by this emerging vampire society, and is set for execution, whereupon he reflects that he is really the 'monster' to them, having been killing them for quite a long time and going back into hiding. He declares before death that 'I am Legend', which makes perfect sense given the context of the story.
It's a shame, because Will Smith is talented, and you could tell that there was a large budget involved. Sadly, both the theatrical cut and the original alternate ending are a terrible distortion of the classic novel.