In the modern era of games, when talking about plot, you get key features, like dialogue, relationships, and development. However, back when Final Fantasy IV came out, such things only needed to be hinted at before the fans eagerly nodded and continued playing on.
The plot, for the most part, just served as a springboard to throw the player straight into the game and keep playing to see its end. Unfortunately, it doesn't hold well by modern standards. Please note that when I speak badly of the game, I am critiquing the "where it stands now" rather than "how it was back then." In all accounts, this game was fantastic back in its time period.
So let's discuss the good; Characters, while somewhat shallow, have quirky behavior that you can fall in love with. I can say the twins were probably a favorite for all fans, new and old, as well as Cid. Kain certainly tugged a lot of heartstrings, and many believe he is the main character. He's even in the logo of one version of the game!
Music is beautiful, typically Final Fantasy epic. Gameplay is also typical Final Fantasy, and it has the attraction to completely the game entirely just so you can get to the special bosses and prizes.
Now for the bad; The storytelling is pretty weak. My biggest problem was with Cecil and Rosa, some of the least developed characters, as while you play as Cecil and see him become a Paladin, there isn't much themes and blatant development going on. I do applaud the DS version for giving the cutscenes some added "oomph" and also the nice feature of the thought bubbles, but really, those needed to be placed in the story itself.
Notice I didn't mention the game style or graphics? Truth be told, since it is one of the older games, I didn't want to consider these to be prime features to focus on. The DS version improves the graphics and style somewhat, but it is still reminiscent of the old version.
Final verdict: if you're a Final Fantasy fan, play this game. It is well worth it. If you're a noobie to the series, you might want to pick it up. Rent it, at least.