So it turns out I agree completely with Ebert & Roeper. This is a great movie and it deserves to be seen in theaters and then treasured on video for years to come.
It's really a classic PG movie, like they used to make in the early '90s. It's about on the level of Honey I Shrunk the Kids (though actually probably a bit more sophisticated). This is a perfect movie for the 10-year-old crowd, assuming the damage done to them by modern kids' programming is still reversible. It doesn't insult kids' intelligence with tons of bathroom humor (and the bathroom humor that is there is just quirky enough) or characters who do nothing more than crack wise and scream at stuff. And they also don't insult adults' intelligence with jokes that say "Are you bored? Here's a joke for you that'll go over the kids' heads!" There are some jokes relating to sexuality and other "mature themes", but they're the kind of jokes that a kid would come up with. It really just radiates with childlike creativity.
The characters are likeable and smart, the story is unique (and relatively free of plot holes, or at least immune to them), and it's all told really well with good pacing and general respect for the story and characters.
So what's the flaw? The animation. It's not really a distraction, and it really doesn't detract from the movie much. But if you have an eye for animation (and most of us do), you'll probably be disheartened by some of the stop-motion technique. It almost seems to have a point in some parts, and it's really not a very "cartoony" animated movie, but I'm almost positive I would've preferred that they did the animation completely by hand. Quite a few of the character movements have a weird jerkiness to them. I think it's been proven time and time again that replicating human motions by hand is more successful, convincing, and flexible than motion-capture. (I think back to 3D wrestling video games -- the first ones used true motion-capture, but they ditched it in the later ones and it did look better). However, because the characters are stylized instead of being as realistic as possible, it doesn't look as creepy as it did in The Polar Express.
It's something that can be overlooked, and you'll probably be able to forgive it, just as you can forgive the animation problems of a low-budget stop-motion film. I do strongly encourage that they ditch this technique, though.
Overall, I definitely recommend this movie. It's a PG classic. And it's the kind of animated movie I've been waiting to see: a NON-comedy family film. It has funny parts, but this isn't trying to be the next Shrek. It's about time.