Not the best in the series by any means, but still possesses everything necessary to make an enjoyable RPG.
First, the bad. The plot is sparse and not especially compelling. For some reason they've reverted to the "silent protagonist" format that is so underwhelming now that we've encountered protagonists with real character and personality. (It's not as if you have any real control over the hero's development, so identifying with him isn't really sufficient as a reason.) It's also distributed in quick bursts with a lot of sidequesting and levelling in between (at least if, like me, you wanted to collect the 108 stars that are the hallmark of the story.) There's also a lot of sailing, and the sailing is both slow and frequently interrupted by the game's crazy-making encounter rate. They've shrunk the party to four characters, and force you to learn the combo attacks previously granted merely by placing the relevant characters in your party. Breaks the import-from-previous-game chain. Some of the voice acting sucks hard.
The good: duels and large scale party battles remain and are suitably entertaining. IV's naval battles are about the right mix of tactics and simplicity. It's very quick and easy to get previously unused characters into fighting shape. Easier still when you get the trainer, and at a certain point it's quite possible to spend about an hour (a really boring hour, but an hour) to max out the levels of enough people to dominate the rest of the game. Collecting the 108 stars remains as fun as ever. There are several minigames of varying quality. (I rather like Rita-pon.) Spell effects are attractive and interesting, and magic is more readily useful than in previous titles due to the ready availability of one's headquarters. The archipelago setting is unusual and lends itself to some original gameplay mechanics, of varying degrees of pleasantness (the sailing needs work, but has a certain cool factor.). The final boss (while made very easy by my max-levelling of my characters) was very cool.