First the cool stuff. The plot centers around a fight to the death between 2 teams with 5 warriors on each team. Here's the thing: each warrior has a unique power which isn't spelled out for us. We need to piece it together what their power is and how they got it. These aren't just a bunch of sword swingers running through the forest; they each have a very specialized skill that they stick to (often not even involving weaponry).
For example, one character is a woman who was raised on poison, and so she has literally the kiss of death. But it goes deeper than that. Having the kiss of death means that she can never know true love because all her lovers would die, and so she leads a tragic life, existing only for the sake of killing though she years to know what love is. How cool is that!
Other characters possess similarly complex powers which lead to deep conflicts in life, and you realize that even the most ruthless ones have an unspoken human side.
The main story focuses on two lovers who, you guessed it, end up on opposite teams. Here is where the Shakespearean question of fate plays heavily. One of them believes that they can somehow beat their tragic fate while the other is resigned to a hopeless ending. What I loved about this movie (at first) is that it doesn't waste time with waffling sensibility: right off the bat, the lovers refuse to fight and they make their peaceful intentions known. But slowly they start getting dragged into the inevitable violence.
But unfortunately the film later falls back on a few clichés which we've seen dozens of times before in other tales of love and war, and that's where I dock Shinobi a few points. With its original setup, I was hoping it was going to stay in that vein and avoid what so many other films have done. It does stay unique, but one or two critical plot elements were pure cookie cutter. Making matters worse, these plot elements were not in line with the characters' personalities. It's like the clichés were thrown in just because the film had to have them.
It's not a fatal flaw, and you may not even notice unless like me you've watched dozens of epic love/war stories like this. Who knows, maybe the whole thing will be fresh and new for you. But I just wanted to temper your expectations a bit. Although Shinobi is a good movie, it's not quite awesome. Still worth your time, though.
Movies in this genre which I *do* consider awesome are the Yimou Zhang films "Hero" (2002) and "House of Flying Daggers" (2004). While "Shinobi" is a great effort, I think you should check out those others first to get a taste of cinematic perfection.