Ponyo is easily the best thing Miyazaki's done since Porco Rosso, largely because it, like Porco Rosso, completely can't be fucked with having a plot. The weight of his story threatened to crush both Mononoke and Kamikakushi, and absolutely ruined Howl's Moving Clusterfuck; Ponyo represents a welcome return to form.
The dub is beautiful done; were it not for a few obvious greetings and utterly arbitrary honorifics, you'd think it was always in English. The two kids get a bit shrill at times, but we had actual five year olds in the theater to compare them to, and they came off well. (These kids were never inappropriately vocal, and one of them happily declared this the best movie ever, so we're cool.)
I do advise you to leave before the end credits song slides horrifyingly into a techno remix from hell - zombie Disney couldn't resist rising from the grave to assfuck Miyazaki's movie at the last possible second.
While I doubt Miyazaki will ever be able to surpass Porco Rosso, I do have to say the scene with Ponyo running along the backs of the giant fish as the kid's mom drives frantically along the winding road is possibly the finest thing he's ever directed.