This was a great end to the series, making really pertinent parallels with the lives of the generation that grew up with Toy Story and ending on just the right, methodically crafted note. It is not, however, particularly funny or engaging for an adult audience.
Don't get me wrong, some of the jokes are great, and the story certainly makes sense, and doesn't resort to cheap tricks or overused elements. It's just that this is clearly still aimed at a really young audience, and there is a lot less of the smart adult humour that I've become accustomed to in recent Pixar flicks. Basically, the jokes I laughed at were things like "Death by Monkeys", and the film only lightly touched upon the more mature themes of the storyline (loss of innocence, moving on, growing old, etc.). In essence, I think this movie found itself in a kind of limbo: on the one hand, the younger audience members couldn't really connect with Andy, who was now leaving for college, whereas on the other hand, I couldn't connect with the maturity of the writing.
Regardless, as a whole this movie is definitely a worthy conclusion to such a popular series. I guess I just expected it to follow its original audience's maturity level a bit more closely.
In a sentence:
A fantastic closure to the trilogy, with some amazing production values, tons of cool details and some really original concepts, but marred by its failure to keep up with its original audience in terms of writing and humour.