The story is simple: The American president is shot, and the story of whodunnit is unravelled through a rewind gimmick: After the movie progresses to a certain point, it rewinds and shows the same events from another point of view. They do this for about the first half before switching to a more traditional style, as if sensing the audience annoyance.
The acting was mostly solid, though the biggest names - Sigourney Weaver and Forest Whittaker - were basically unneccessary. Whittaker's character especially was basically irrelevent: most of his interactions with the main players were not mandatory to the progression of the plot, and the sub-story with little girl Anna was completely beside the point.
The movie implausibly takes place within a few blocks of the assassination, so that no matter how long the car chase goes on, other players are able to stay within distance by running. In several places a speeding car is unable to get away from a man on foot. No matter how well executed the meat of the movie is, you can't help but feel it's a 20-minute story stretched out to movie-length with clever edits and a POV gimmick.
I don't mean to sound too negative - there are good points throughout - but ultimately it's a forgettable flick, and your two hours would be better spent elsewhere.