This was one of those movies that get graded based on the performance of its lead since they are going to be the only thing on the screen for huge chunks of time. Like Tom Hanks in Cast Away or Will Smith in I Am Legend. Reese Witherspoon's performance was definitely as good as those and that made this movie and her journey an excellent watch. You felt like you were there with her and felt all the same emotions and pain. The way that they told her backstory in flashback form depending on what was going on during her escapade kept you very interested and wanting to know more.
I would definitely recommend people checking this movie out. It was a great work of storytelling with an incredibly wide range of emotions. Reese Witherspoon will surely get nominated for several awards because of this performance - and rightfully so.