Much more amusing than expected!
OK, so you've seen Groundhog Day, and Edge of Tomorrow, and Frequency, and a dozen other time travel/time loop movies, and you think you've got the idea.
Well, you're probably right. The basic structure is easy to understand: Start each day, proceed until death, wake up and re-live the day with the knowledge of what happened before. But there are a few twists and wrinkles that keep this movie from becoming tedious, like these types of movies occasionally do.
I actually liked the fact that the film didn't try to explain too much about how or why the loop is working - those things weigh down time travel stories with exposition and ultimately result in absurd plot problems anyway. This movie focuses on peeling away at the lead, rather than dry scientific explanations. There are some hints about the rules of the loop, but ultimately this movie is concerned with its lead character, not its technical time travel foundation.
Jessica Rothe has a really strong screen presence that keeps your attention fixed as she progresses from smugness and indifference, to shock and confusion, to fear and pain, to anger and frustration, and finally to determination and confidence. With a little happiness, sadness, and flirtation mixed in. It's a really demanding performance that keeps her on screen for virtually every shot in this roller coaster of a movie, but she's up to it.
So it's not as deep as Primer, not as mind-bending as Donnie Darko, not as gritty as Looper. But as much as I admire those films, I would say this movie is easily as much fun to watch. If those films are fine steak dishes, this one is a really well-made hamburger: A lot faster to devour and digest, and pretty damn satisfying in its own right.