I did not feel that the movie was that bad – there were a couple of discrepancies in the script that caused it to be a lot less believable. However, the acting on behalf of Ashton Kutcher was actually remarkably good, especially notable given that he is best known for his role on 'That '70s Show' as Michael Kelso. He and the rest of the cast put in some very believable performances - each one really had to act multiple characters given that things were so different in each different timeline.
The main plot discrepancy which stood out in my mind was when he was in the prison, he managed to get a hold of his journals by convincing one of the inmates that he spoke to God – he told the inmate that he should watch out for anything that changed after he read the page of journal which he did posses. So he went back in time to when he was in a classroom, and smacked both of his hands down on the big spikes on the teacher's desk. Coming back to the present, the inmate thought that the scars on his hands were stigmata – it occurred to me that the prisoner should have remembered having seen them all the while rather than believing them to have suddenly appeared.
As for the ending of the film, there seems to have been two. Here in Singapore we got what seems to be the “family-safe” version – the film ends with Evan watching film footage of him meeting his girlfriend-to-be for the first time when he was 8(ish). He tells her something along the lines of “if you ever come near me again, I will kill you and your entire family.” The little girl was of course afraid, and therefore when she was given the choice to either stay with her father or mother she and her brother chose the latter, thereby Evan never had to worry about killing/hurting her.
The other version included a gypsy and footage of his mother giving birth (or so I hear, I never saw this version myself.) He is told that his father used to see this gypsy fortune-teller all the time, and when he goes to see her he is told that he was never meant to be born. So in the film's conclusion he watches footage of his mother giving birth, goes back in time and chews through his umbilical cord whilst still in the womb. None of this seen in the version I saw, no gypsy was ever mentioned - I am not at all surprised that the Singapore film censorship board chose the other version.
All in all, a good concept I would say. The film was hurt by the fact that many members of the audience couldn't put away their preconceptions of Ashton as a non-serious actor – we all expected him to say funny things, and because of that they were; kind of similar to the whole “Agent Elrond” thing in Lord Of The Rings.