My roommate just bought this game and I tried out the first level with him, playing co-op. I certainly don't hate the game (I really like the graphics, and I like how they really delve into the story), but the on-rails thing is a real turn-off for me. I like being able to explore a bit and get lost and screw up and such, and so far I'm missing that experience here. I left him to continue playing on his own to see what he thought of it (since he bought it), and I'm sure I'll play more of it, but I'll just echo the comments already stated above, that the game would've been better had it not been on-rails. I know the idea of the 'mature' game is a fairly hard-sell on the Wii, but it clearly has been done a few times with success. Is the Wii THE platform for 'mature' rated and first/third person shooter games? No, but it's a viable platform for those experiences nonetheless.
Bah!
