I left The Muppets with a smile on my face, a grin that had been there for most of the movie. In truth, I can't remember many of the songs from the movie, but it doesn't bother me too much. It wasn't exactly a "Muppet" movie in the way that The Muppet Movie, The Great Muppet Caper, or Muppets Take Manhattan*, story-wise, although it nails the self-referential nature of the first three films and really understands how to use a well-placed celebrity cameo.
This is due in large part to the screenplay by Nicholas Stoller (Get Him to the Greek) and Jason Segel (Forgetting Sarah Marshall), who are clearly Muppet fans looking to make a film that did them justice. With director James Bobin (Da Ali G Show, Flight of the Conchords), they put together a goofy, clever, self-deprecating film about two brothers, Gary (Segel) and Walter (Peter Linz) who grew up loving the Muppets. Walter, in fact, idolizes them, which may have something to do with the fact that he IS a Muppet, although the film never directly addresses that point.
Gary, Mary, and Walter set out to find Kermit the Frog and reunite the Muppets to stop Tex Richman from winning, and hijinks ensue. They gang needs to clean up the theatres, convince Miss Piggy to come back, and find a host for the show - all of which are addressed in amusing ways. I'm not going to tell you who the celebrity host is (mandated by a TV Executive played by Rashida Jones), in part because the person who plays them is the only celebrity not credited in the film, despite the fact their name is mentioned repeatedly. I'm also not going to spoil the many cameos in the film, aside from a well placed appearance by Zach Galifianakis as "Hobo Joe" near the end.
(I was sad that Steve Martin isn't in the film; the closest we get to seeing him is on a photograph in a dressing room.)
Now, to the songs... you see, it IS a musical as the film leads up to The Muppet Show telethon (where there is, of course, a version of "The Rainbow Connection"), but I can only remember two of the songs (written by, I think, Segel with contributions from Flight of the Conchords Bret McKenzie): "Am I a Man (Or Am I a Muppet)" and "Party for One." The former sticks out because of the Muppet version of Gary (and the human version of Walter, played by The Big Bang Theory's Jim Parsons), and the latter because it's a very silly disco song with Amy Adams that eventually includes Miss Piggy. Other than that, I know there were other original songs, but none of them stuck with me. That should bother me, but the film is so engaging in other ways that I let it slide.
I'm debating whether or not I enjoyed the film because it was lovingly crafted or because of some level of nostalgia, which seems to underscore nearly every other review I've seen online. It is true that if The Empire Strikes Back wasn't the first movie I went to, The Muppet Movie was, and I do still watch The Muppet Show regularly. I understand that attachment that Gary and Walter have to the show and the movies (although it's unclear if Gary and Walter saw the movies, even though Kermit mentions them).
That said, I haven't had the same attachment that friends who are slightly younger do the The Muppets. I didn't see The Muppet Christmas Carol, Muppets' Treasure Island, or Muppets from Space. I watched the "Bohemian Rhapsody" video on YouTube and chuckled, because it did seem like The Muppets that used to be, rather than the adrift, post-Henson era Muppets, but I was excited to see a Muppet movie as and adult. So were my friends, unabashedly. The folks I saw it with had already seen the film and had no qualms about watching it again. I don't think I'd hesitate to watch it again, either, and I recommend it to families with children that are maybe tired of "kiddie" movies that test the patience of adults. It's silly, to be sure, but I don't think you NEED to have been a "Muppet" kid to enjoy the film, even if it's clear that Segel and Stoller wrote the film as a love letter to Henson and company. Chances are you'll chuckle quite a bit, smile a bit more, and be happy to spend your time with a mostly proper Muppet movie.
P.S. Despite his presence on the poster, I really don't think Rizzo was anywhere to be found in the movie, so fans might be a bit let down by that.
* I understand that there are three other Muppet films released between Manhattan and The Muppets, but I've never really considered them when discussing the series. It's largely because they followed Jim Henson's death and other than Muppets from Space, they are also more "Muppets in ___ existing story" films.