How others will see it. With one exception, this is grand entertainment for anyone who has a sense of humor and tends not to sneer at mainstream Walt Disney fare. Those who remember the Muppets circa-1980 syndicated television series are in a better position to enjoy the cameo appearances of minor muppets. They're all present and accounted for, except for the piano playing dog, who isn't missed.
What's the exception? Young Hawkins is in nearly every scene, since he is the protagonist stand-in for the youths who read the original novel. Althogh mothers may coo at his pretty early adolescent face, his continual presence may annoy those who want their Muppet anarchy unalloyed.
How I felt about it. Muppet Treasure Island has had little cultural impact. It isn't taken seriously, actually a mistake since quality silliness is vastly underrated. Its principal fault is that it didn't make a killing at the box office, and the series of full cast Muppet movies faded out after Muppets from Space (1999).
Other than The Muppet Christmas Carol (1992), Muppet Treasure Island was the first Muppet feature made after the death of Jim Henson, the Muppets founder, who voiced Kermit the Frog back in his pre "Sesame Street" black-and-white television days. The loss of Jim Henson means a loss of the spiritual soul of the Muppets. But his son, Brian Henson, directs, and longtime Henson right hand man Frank Oz is onboard. The Muppet characters remain true to their Muppet show counterparts.
Their cause is aided by a nifty set of songs composed by Barry Mann & Cynthia Weil, who wrote numerous Top 40 hits during the 1960s. No doubt, Mann and Weil put everything into the project, hoping that it would lead to a major career comeback. That didn't happen, but the songs are nonetheless pretty good.
Beneath all the silliness are plentiful well-placed zingers, present to let you know this is a Muppet Treasure Island rather than the more gripping pirate tale. Before launching into a song, Tim Curry confides, "This is my only number." A jailed Muppet pirate receives bread and water, and complains, "But I ordered shrimp scampi!"
In short, it's better than you've been told, and it's better than you think it is. And you should give yourself permission to simply enjoy it.