Alright, so let's just get one thing out of the way. Even though this is a time travel story, it's not science fiction. There is absolutely no attempt to scientifically explain the time travel; in fact, the means by which the time travel (called replays) occur to Jeff Winston and others are intentionally never explained in any way, scientifically or otherwise. The replays are thus a plot device, a means by which Grimwood presents the ideas he wishes to have the reader ponder. In this way, explaining how the replays happen would have spoiled the story, as the focus of the story is really on how and why Jeff is living the way he is. To keep the focus on Jeff's mindset, the time travel had to remain something mysterious, something magical. So, fantasy it is.
Although the book follows the many lives of Jeff Winston, it's really about more than that. Winston tries out many of the classic time travel tropes in the course of the tale, including using his knowledge of the future to get rich, attempting to alter historical events, seeking out experiences he hadn't had in his previous life, and so on. The only time travel trope he seems to avoid is attempting to kill himself. What makes this story interesting is that Grimwood shows the inevitability of the thought processes which lead Jeff to make these decisions, and the core of the story is really the same as in many other stories - Jeff is trying anything he can think of in his quest to discover the meaning of it all.
Grimwood cleverly has the reader ask questions about their own lives, questions which truly have no answers. The moral of the tale is a simple one, albeit still fairly profound; that life is what you make of it, and regret over mistakes made, though understandable, is ultimately futile.
I would recommend this novel to anyone, not just genre fanatics like myself. It transcends genre, and I'm surprised it's not more well-known.