I've played my way through the DS version of Final Fantasy IV up to Cecil's transformation on Mount Ordeal.
Thus far, I've been impressed by the amount of plot material and the careful attention paid to each member of the cast. The later is something that I felt Final Fantasy VII occasionally fell flat on. I also enjoy most of the heroic fantasy melodrama and find that it's worn well over the years (perhaps an updated translation of the dialogue helps with that?). Rosa needs a serious feminist intervention, but her kindness toward Edward and Rydia give her a little counter-balancing depth to all that save-me-save-me-Barbie-princess non-sense. That aside, every character has a well-established motivation and background that fits in well with those of the other characters and the overall plot.
Another difference from Final Fantasy VII (the game in the series that I've played through most recently) is that each member of the ensemble has very distinct skills that have to be used in tandem in order to succeed. Since characters come and go over the course of the plot, the player has to quickly adjust to working with different combinations of skills. Furthermore, each character's stats can only be raised to certain levels that vary from character to character. I have to say that I like working with these set characters rather than having every character be widely customizable and capable of near-infinite progress. It gives a greater sense of constraint to Final Fantasy IV and makes it more challenging than VI and VII.
Those are my notes so far. I'm enjoying the game and will report back with more thoughts as soon as I have covered some more ground.