Before watching the movie:
For my first review, I decided to watch Simon Birch. This movie was indirectly recommended to me by my literature class when we read A Prayer for Owen Meany, which was the basis for the movie. Many classmates had seen the film, and some commented they couldn’t help but read the narrative in Jim Carrey’s voice.
I feel a little like I’m cheating by reviewing this movie, because I’ve read the book and have a general idea of how it goes. But I also know the movie was rejected by John Irving as an adaptation, which is why it has none of the character names, and I know the end is superficially changed in some manner, so I don’t think it’s any worse than a movie I’ve seen a bunch of trailers for, or heard people talk about. It’s certainly less familiar to me than some I’ve been so interested in that I read the entire plot online. As an adaptation of a book I’ve read, I’ll appraise it as such.
After watching the movie:
The first act feels like a very faithful adaptation. For the first half of the film or so, I was feeling guilty for picking it, as so much of my discourse is comparison of the book to the movie.
Seeing it on the screen really drives home to me how much Simon/Owen is entrenched in Joe’s/John’s family. I was slightly disturbed by it in the book, but in the movie it’s almost creepy how much Joe’s mother cares for Simon and how little Simon’s parents are present. In contrast, I completely do not believe the mother’s death in the movie.
I’m not sure what to make of giving Simon/Owen a heart defect that makes him a miracle up front. Owen is asserted to be a miracle child, but few people know it. Simon doesn’t let anyone forget it. The heart defect itself does mark him as destined for something, like the main character in Gattaca, which is why I have mixed feelings.
The characters comment on Simon’s voice (a “shriveled mouse”), but while he’s a child, I’m not hearing it. I’m a little disappointed they didn’t make a better attempt, but I’m more disappointed that they left it as an informed trait.
Simon’s lines are written very well, even many of the ones not copied from Owen Meany, but his delivery is much like most child actors. The effect is that I buy him as a sage, but only just.
Oliver Platt’s performance as Joe’s stepfather doesn’t really match what I had in mind for his character. He’s just as genial and wise enough when he needs to be, but I expected him to seem more intelligent. Maybe it’s that I consider him physically wrong for the role. On the other hand, the only thing I’ve seen him do is dance in comically oversized pants on a kid’s show.
In the book, the stuffed armadillo is only important for the symbolism Owen imparts upon it. In the movie, that symbolism is gone. All that’s left are two memorable but not plot-oriented scenes.
The manner Simon ruins the Christmas Pageant is entirely different from the book, and I don’t like it. Owen manages the perfect pageant, then blows it by scolding his parents from the manger for attending. Simon gets excited because the girl who plays Mary has her breasts right in his face, and drags her into the manger. “Sex makes people do crazy things,” but that’s just absurd. The pageant degenerates into mad antics after that, which is probably why they did it.
After the pageant, the third act is crafted almost entirely of whole cloth. This shrinks the scope of the film considerably. Simon’s independent views on faith have run their course, and except for his belief in his own purpose, have nothing more to do with the plot.
Trying very hard not to spoil the end, I make one final comparison. Everything about the end of Owen Meany is sudden, immediate, moving, and organic. Conversely, everything about the end of Simon Birch is prolonged, telegraphed, frustrating, and contrived. And overall, I would have to say that the movie itself is telegraphed and contrived. Everything that is great about the film is from Irving’s book, and everything that is terrible about the film is foreign to the book.
See this movie: if you want a slightly Christmassy feelgood cry.
Don’t see this movie: if you’re a fan of A Prayer for Owen Meany. The results of the adaptation will make you cry.
Yesterday's Movies 
No, the cheat is a producion team who use an existing story like raw lumber instead of just telling it. It sounds like as the work progressed, somebody said, “oops, we’re not following The Formula” and started shoving in Standard Movie Story Features.
Sadly, so many stories would have been so much better if they had been told by storytellers, not committees of industrial psychologists. In many cases, most of us will never know how much better.
It’s been so long since i read Owen Meany that i hardly remember it. The part you mention about how Simon ruined the pageant reminded me of another “yesterday’s movie” you should watch. If i remember correctly, it’s called Sweet Liberty; it starred Alan Alda as a history professor frustrated with what the movie producers were doing with his serious bestseller.