Watching Memento is like putting together pieces of a jigsaw puzzle. Not only is it that way for the viewer but for Leonard as well. By having the narrative structured the way it was it allows us to be in a similar situation as the main character. The story seems to be told backwards. This is done well by showing a scene and then immediately showing the scene that led into it in the story.
Flashbacks are also used but at times it is hard to tell when the flashback took place. Most of the flashbacks seem to use black and white which does make them more obvious as flashbacks. Time for Leonard now means nothing. He has no idea how long he has been like this. He isn't even really sure what memories are his or which ones are even real. The photographs are his puzzle pieces. He doesn't remember taking them or writing on them and must rely on them as facts.
At the end of the film we are still questioning exactly what happened. In a way it is fitting since Leonard will be facing this battle the rest of his life.