The first of the official Ghibli films, Laputa is most similar to its predecessor Nausicaa, but whereas Nausicaa was a SF epic, this is more of an action comedy-adventure with a fairly weak SF premise.
For the first half hour of the movie I thought I was going to love it. Once again you find yourself in awe of Miyazaki's attention to detail, and his ability to conjure up an imaginary world in meticulous, beautiful perfection. The animation, though still not in the league of the later Ghibli films, is a little better than in Nausicaa.
I mentioned in my review of Nausicaa that one character is drawn in a more 'Lupinesque' anime style. Here there are a whole bunch of them: the pirates. They provide the comic element in the film, though quite why it needs a comic element I'm not sure. There was nothing funny about Nausicaa, and I think it was better off for it. Still. having said that, Dola, the pirate leader, is easily the most memorable character in the film (even if she's basically a female Long John Silver. Don't be surprised if you're reminded of 'Treasure Planet' at times) and is well voiced by Cloris Leachman in the American dub.
The English voice cast acquit themselves well for the most part, actually. However something started going pear-shaped for me about this movie by about the halfway mark. I think the best way that I could describe it is that the characters get swallowed up by the vast scope of the story. I'll come back to that later.
There is so much to admire about Laputa, and so many people obviously love it, that I almost feel mean for only giving it 8 out of 10, but for me the operative word here is 'admire'. It was more impressive than personally affecting, and at over two hours it just started to drag.
The crucial thing for me was this: I really found that I didn't care much about any of the characters. Disney would have taken the care to develop the characters, make you really fall for them, rather than leave them as relatively two-dimensional pieces to be moved around while you gawked at the amazing vistas in the movie. Even that would have been tolerable if there was some hard SF in the story to make up for it, but it was basically a lot of gobledegook about Princesses and magic crystals. That's where Laputa falls down as far as I'm concerned, and that's what holds it back from being a potential 10 out of 10 movie. Having said that, it must be admitted that Miyazaki was leagues ahead of Disney in general in 1986, when you consider that their movie of the same year was the pretty dire 'Great Mouse Detective'.
Laputa is a good film in some ways an amazing film, and you should definitely see it, but I do feel it's over-rated. I definitely prefer the earlier 'Nausicaa of the Valley of the Wind'.
BTW, if you manage to watch them in chronological order, watch for the fox-squirrel from Nausicaa popping up briefly in Laputa.

Watching this movie is like eating a banquet of nothing but meringue. It initially looks great but ultimately provides NO satisfaction--none.
The plot is a muddled mess about a toy factory and the forces of evil. So, how is it possible that with this basic plot AND Robin Williams that the movie still turns out so badly?! It's because the picture is all appearance with no substance whatsoever--much like the terrible Popeye picture Williams did at the beginning of his film career. The film must have cost a fortune but perhaps there wasn't enough money left over to hire writers who had graduated grade school.
The film is one unfunny joke that goes on and on and on and on. I really am unsure why it was made in the first place--it certainly wasn't made to provide any sort of entertainment.