The Legend of Zelda is a fantastic series, but too much of the same isn't always a good thing, even if it works.
On its own merit, Phantom Hourglass is a pretty good game, but it's definitely the least enjoyable of the Zelda series. It's the same formula as the rest of the Zelda games, only more simplistic. And let's face it, the character sprites are appalling. They remind me of the Wii avatars. Ocarina of Time had much better character graphics, which made PH a disappointment in that regard considering the time they had to improve on it.
I'm still giving this game a very high rating though, as the gameplay itself wasn't bad. Controls are smooth (although it would have been nice to be able to control Link with the actual buttons), and the scenery/enemies have decent graphics compared to the character sprites. All in all, it's definitely a Zelda game for younger kids, but it's still fun to play for adults who aren't too sick of the standardised formula yet.