Well, I cried. The Rainbow Connection gets me every time.
As far as corny kids musicals go, this is fantastic. For adult viewing, if you can put up with a very Sesame-Street feeling for the first 20 minutes, it's surprisingly nuanced. All the major muppets are so much more complex and dynamic than I remember, in positive, emotionally mature ways. Piggy is shown to have learned from her terrible temper; Kermit seems to open up more than I ever recall. Animal learns a little bit of balance in his life. Gonzo and Fozzie are still pathetic, but hey, you can't have it all.
I have three big concerns about this movie: the violence, the status of muppets, and the consumerism. I'll address them in turn.
The violence is fine for most kids who are able to reason, but the very young ones could come to some very nasty conclusions here. There's a mildly violent kidnapping, some intentional fighting, and some serious vandalism, but it isn't the acts themselves that concern me. It's that they get away with it. The kidnapping in the film goes unpunished except for chastisement from Kermit, and an accidental violent act later acts as a sort of deus ex machina for the plot. Young children may not be able to process that violence is wrong when they see someone get away with it, so be mindful of your little ones' moral development.
The status of puppets in the world of this movie concerns me a little. Walter is portrayed as a misfit and almost disabled for being a puppet, and his actual genetic relationship to his brother is unclear. Is being a puppet like having a disability? Are random people born into flesh families as puppets and subsequently treated badly as if they were another class of people? Talk to your kids about what it means to be different and whether being different is bad when you watch this. Even the older ones. Please.
Lastly, money and wealth are equated with both tremendous power and freedom and horrific restriction and shame. While a balanced, nuanced look at rights in the arts, the plight of washed up performers, and the difficulty of balancing wealth and freedom, this may be too hard for young kids to understand. They may just see all the shiny bling and costumes and decide that money and fame are good, without realizing what it cost for characters like Fozzie, Gonzo, and Piggy. The muppets and villains both commit serious crimes for money. You might want to nix any associations of kidnapping and money in your little ones before they take hold and discuss it right after the movie.
This is a great movie for all ages, but it could use a little aftercare in those whose morality still consists of avoiding punishment or being a good boy/good girl.