Pretty much on par with the first film. That means it's a little better than most US CG animations of that time, but hardly worth recommending, unless you're looking for an age-appropriate film to show your toddlers. Apart from that, I think the target audience for this film is pretty narrow.
Stuart and his brother George are slowly growing apart. George has made friends of his own, which leaves Stuart without anyone to play with. Until he meets a bird (an actual one, though she's also female), who literally drops into his little car. The two hit it off, but the bird is hiding a secret.
The animation is cute, the designs and colorful and charming, but the rest of it is extremely safe, mushy and clean, to the point where it can get a little sickening. Characters are dull, Stuart's adventures are quite lame and the drama is way too geared at US morality. It's decent for the little ones, I like a bit more edge, even if it's just children's entertainment.