I love the movie, but I think a great deal of that love has to do with how old I was when it was released. I think it represented the perfect fantasy for me, and for many of my friends at the time. Rainy afternoons, gadgets, treasure maps, booby-traps, adolecent sex-teasing, and pirate ships. And mounted by Donner's sure hand it never takes itself TOO seriously, but doesn't play as parody either. I do remember a bunch of us pulled out our old running shoes and converted them to (working) slick-shoes.
Watching it now, parts of it don't hold up especially well, but the interaction between the kids is genuine (something you hardly see with kid actors today) and fun to watch.