Single player is no where near as bad as some people claim it is. Since there are no random battles "leveling up" is achieved through completing areas again, which gets you an artifact. Each artifact gives you a stat bonus, extra health, extra command slots (You start with 4, 2 of which are taken up with Attack and Defend) or other bonuses. This is certainly different from most Final Fantasy games but suits a mainly multiplayer action orientated game better than a complicated experience system.
Multiplayer is what this game is all about though. Working together to complete each area is great fun and spell fusion makes casting spells together require coordination between players. I have only played it with two players, but I've read that the more players the more fun it gets. To keep things interesting there is also a bonus point system each time you do an area, where each player gets told something they need to do and the better they do it the higher score they get. The player with the highest score gets first pick of the artifacts at the end of the level. Tasks include "taking damage" and "do not pick up items". These add to the variety of levels and help add a little competition into an otherwise co-operative game.
The plot of the game is also moved along by semi-random events between areas which while aren't as deep as other FF games it seemed fine to me (and even made me laugh in places). Since the game is not based around specific characers most of the plot is based around characters other than your own. Some character interaction between your own groups character would have been nice but since it can be played with only one character aswell as up to 4, it's not something which could have been done easily.
Earlier this week I almost completed the game (at least I think I did) but the final boss took me so long, and I died too easily so decided to "level up" some more before trying again. I plan to try and get all the items and artifacts eventually once I have managed to kill the final boss.