I picked this game up last night. I have played Final Fantasy 4 [and 2, when it was called that] more than any other of the series. I found the game vibrantly different from the original in the graphical sense, several exploits within the game have been covered up, and the polish of voice acting and real cutscenes is refreshing as well. The gameplay has changed little from the original and the standard never run away = always win plan works as well as it always has.
For those who have played a Final Fantasy game you will see the standards of the series here. Crystals, objective evil, bad dialog, and a linear story all make their mark here as well but going into a Final Fantasy game you should know what to expect by this time. Active Time Battle makes the game worth paying attention to, an Autobattle system proves that AI still has a long way to go to be useful in games, and the spell/summon system works in a simple drop down menu function like it should. If you've played any of the series prior to X you should have a good idea of what's contained.
For those of you who haven't played the series at all, or any RPG for that matter, I will sum it up for you. If you've never tried an single player RPG because Azeroth takes up too much time, please step away from the computer and go find something fun to do outside. Then try this game as a halfway point between being tethered to your keyboard pressing the same five hotkeys over and over and actual interaction with RL.
Those who don't play RPGs because reading sucks, you are most likely not going to be on a board online posting or at least, not going to be reading what I have typed, due to the amount of polysyllabic words within. In fact IF anyone who has never played RPGs is reading this and made it this far, you will most likely find the game worth playing but you are a weird creature and should first turn yourself over to science for study.
Overall I would say the game is a must for those who's first Final Fantasy game starred some spiky haired effeminate with an overcompensation problem, just for a history lesson. Those who are old hat to the series will find enough "tacked on" to the game to merit a play through to determine if the game is truly worth it to them. And those fanatically devoted to the game [you know who you are] will not be able to sleep until the game is comfortably driven into the warm moist[?] cavern of your DS slot.
The minigames and Whyt summon give the game enough of a connectivity feature that I think the game somewhat uses the DS's powers beyond graphics but those looking for a new touch gimmick should look elsewhere.
I won't bother rating the game on a numerical scale, since I feel games are too nuanced to categorically rate them by a strict number system. Some will enjoy the game based on the reasons posted above and others will ignore it for the same reasons.