It's an odd, rather underrated Hayao Miyazaki film that doesn't really feel like him. It's full of scenes and ingredients that are very Miyazaki and the film's certainly full of his charm, but the film as a whole feels lightweight. It's an adventure. It's not really about anything and it's almost going out of its way to avoid saying anything. It's set in a defined historical era with no fantasy or SF elements, except for Porco Rosso himself.
That said, though, this makes it more straightforwardly entertaining than one of his preachier films like Princess Mononoke. Porco Rosso is a flying pig. (He used to be human, but someone cast some magic on him. No one really cares about this and indeed it's not unknown for women to find him attractive. Since this is set in Italy in 1929, when the country was ruled by fascists, Porco's porcine nature might be a statement in itself.) He used to be a World War One fighter ace, but these days he's a bounty hunter who preys on pirates. He refuses to rejoin the Italian air force ("better a pig than a fascist"), but he's no altruist. He demands cash and lots of it.
That said, though, this starts making more sense when you see the state of the country. When he tries to get his plane repaired, he's told that money isn't worth the paper it's printed on these days. This is a Girl Power film, partly because the depression means the men and the jobs have all gone.
Porco gets more lovable as the film goes on. He picks up an adorable seventeen-year-old mechanic called Fio, who rejects all his gentlemanly efforts to leave her behind. (He's her first customer!) He fights an American ace partly to defend Fio, after she makes an inadvisable bet.
...and that's it, really. It's a lot of fun. The pirates are silly and lovable, e.g. being rendered completely ineffectual by taking fifteen very small hostages. Porco starts out as a borderline anti-hero, but he's not that bad really. It has Miyazaki's love of aviation and his obsessive eye for detail. (The historical recreation is excellent, especially with the planes.) Admittedly the ending's more complex than expected, e.g. averting romantic cliches, but even so this is a light, fun film. The juxtaposition of the setting and the plot makes me want to rewatch it and think some more about it, though.