This is one of maybe 2 or 3 games on the XBL marketplace that is a must buy! The extreme attention to detail is mindboggling.
Let me start by paraphrasing from an early section of the game. The whole theme in this game is that people learn from their mistakes. The only problem is that your mistake has consequences. What if, just *what if*, we had the guts to forgive and forget those mistakes. We would then be rewarded for learning instead of being punished for the mistake. THAT is the key that makes this game so interestingly unique.
In case you haven't heard, the way this game implements such ideas is by allowing you, at any moment, to rewind time. Much like the game "Prince of Persia" except at any time and for as long as you like. This gives you the ultimate freedom to "retry" mistakes you have made.
Of course, with such great power comes great...challenge. This game may be not be for the weak minded. I can honestly say that this is the most fun AND most challenging game I can ever remember playing. That being said, I think I beat it the third day I had it (maybe 3 hours of playtime) in between school, work, and sleep, and that includes every achievement except the "Speed Run" (beating the whole thing in 45min. This also gives you an idea as to HOW short this game can possibly run).
I'll be honest, the game has one of the most incredibly confusing storylines I've ever tried to keep up with. Half of it may be that I underestimated it from the beginning and wasn't really keeping up, but once I caught on, it was a very interesting theme. One of the most interesting parts, however, is that technically I don't think you NEED to solve a single puzzle (thus acquiring a puzzle piece) to actually finish the game. The ending sequence is also very impressive as it ties in the whole "Rewind" feature into the story. You might catch on rather quickly, but pay attention to the order that you unlock worlds ;)
Each world has a new "rule" to it, for instance: In the first world you are permitted to enter, you are under basic rules. You screw up, you rewind, you do it right. In the second world you enter you will notice that some things are allowed to ignore any rewinding or other time-altering effects put on them. As you progress the rules change dramatically, and this leads to a very complex, unique, and challenging game.
Bottom line: Buy this game. It's totally worth $15. You won't find another game like it anywhere, at least not yet.
If you like games that take some thought you will love it. If you're not good at these games, you need the practice.
This game is to video games, what an epiphany is to life. It'll screw with your head big time.