I don't think I liked it. It is surely my least favorite of all the films Hayao Miyazaki has directed, and I've seen all of them, including the pre-Ghibli The Castle of Cagliostro. I've yet to see Isao Takahata's My Neighbors the Yamadas or Goro Miyazaki's Tales from Earthsea, but Ponyo is also so far the only Studio Ghibli film that I haven't really liked.
There is still much to admire in Miyazaki's craft; the film is visually arresting, with resplendent images of the sea and crazily imaginative designs (maybe excessively so for my tastes). But I was incredibly bored throughout my viewing. Not a whole lot seemed to happen in the movie, and there was no sense to anything that did. Whether it was mundane life moments or fantastic spectacles, it all seemed to me just self-indulgent on the part of the animators. Despite an English-language cast that included Liam Neeson and Tina Fey, the characters all came across as really flat to me. I can't believe I'm saying this, but at times I found myself wishing for some sort of wisecracking comic relief animal character to show up and start annoying the hell out of everybody. At least it would have provided a big and consistent personality to grasp onto.
I had been warned that this would be more of a children's movie, compared to the more epic feel of Miyazaki's other films that have made it to US theaters (Princess Mononoke, Spirited Away, Howl's Moving Castle), but I expected something closer to My Neighbor Totoro, which used fantasy to cope with some sad and scary themes. When I watched that for the first time as an adult, I liked it a lot, and I even felt sorry that I had not been able to view it earlier, because I was certain that my kid self would have loved it. I can still kind of channel that kid in me to enjoy many children's entertainments, but I don't think I would have found Ponyo very diverting even had I seen it as a child. I was surely more patient (with movies), but I still hated Fantasia as a six-year-old. I think I would have found Ponyo similarly too slow, too weird, and devoid of any kind of tension. I would not have cared that it was a technical masterpiece, nor would I have found it any less impossible to root for a love between a five-year-old boy and a five-year-old magical fish thing. Most likely I would have walked out and watched The Little Mermaid again instead.
Actually, come to think of it, I didn't love Nausicaä either, so maybe my opinion means nothing.