This adaptation of Jules Verne's novel whose plot will hardly need any synopsis is - like many a movie featuring the great Ray Harryhausen's stop motion animation - a childhood favourite of mine, so any idea of objectivity goes right out the window. However, I don't think Cy Endfield's movie actually needs the nostalgia factor to deserve praise.
After all, this is a film that begins with a rousing balloon escape, turns into a Robinsonade (and the kind of self-conscious Robinsonade that mentions Defoe's Robinson Crusoe to boot), shows off some dangerous giant stop motion animals - a crabby crab, a rude flightless bird, some peeved bees and a grabby chambered nautilus -, sinks a pirate ship, meets Captain Nemo (Herbert Lom), destroys a sunken city, blows up a volcano, and even finds time to invent what should be a steampunk fashion staple in form of the shortest goatskin dress of the 19th century; all in just 110 minutes of running time, directed by Enfield with a sense of excitement and an enthusiasm for the adventurous incident you don't get to see every day.
Somehow, the film even finds time to be silently progressive: Neb Nugent (Dan Jackson), the black member of our group of heroes, may not have as much agency as one would wish for looking back from times when this sort of this has become important, but is still treated as an actual person whose opinions and emotions are respected by his companions without any condescension, something that was not par for the course in 1961; the English noble woman (Joan Greenwood) is much more practical than her position in life or (again, in an adventure movie in 1961) her gender would lead to expect; in general class, gender and race lines are permanently being overstepped by the characters without it elucidating any comment, with the unspoken subtext that rational beings will overcome such artificial divisions when they have been given the opportunity to. One might find the film's politics naively optimistic, but if we don't even allow ourselves to dream of the improvability of humanity in our SF adventure movies, we might as well all step in line and pray to our corporate overlords. And isn't it a fine irony that Mysterious Island was in fact financed by some of those very same corporate overlords? But I digress.
On the level of pure filmmaking, there's little to criticize about Mysterious Island: Harryhausen's effects are pretty much perfect; Endfield's direction tight in that effective way that has no room for showing off and keeps brilliant direction all too easily from being called brilliant; the script is imaginative and more complex than it has any need to be; Bernard Herrmann's score is rousing and playful in turns. If I needed to find fault with something, it's probably the acting of Michael Craig and Michael Callan, respectively the movie's square-jawed hero and the teenage heartthrob, but they're not that bad, really. The rest of the cast fills their archetypal roles admirably.
So yeah, Mysterious Island. Watch it. It's still awesome.