Linear does not equal bad.
But let me tell you, this game does not deserve the many criticisms it has. It has flaws, but so does most games that everybody love and praise, and in the big picture, it's better than some of them.
To get it out of the way, i'll talk about the linearity in this game, it's true, the game is basically a linear corridor and a big room to do sidequests, optional uber-hard bosses etc.
Everything is more easy to manage than in previous final fantasy games, more simplistic if you want.
But this does not have the consequence everybody assumes, the game is not easy at all, it's the hardest final fantasy i have played(6 through 13), because it forces you to think and change paradigms on the spot, and also be smart using the most powerful abilities, which have a limited use for each battle.
There are some flaws in the gameplay in my opinion, like the inability to change the party inside a combat. Is not like in final fantasy X, where i played with all of the characters, in this one i choose three and stuck with them.
In the same way you are also stuck with one summon, the one your favorite leader has, i didn't even see some of the summonings.
But there's also a very positive difference with other final fantasies, although this may be very personal, it's the fact that i liked all the characters, and i found them all to be complex and engaging.
The story is very good in the way it's implemented, without barely any exposition, it lets you pick and join the pieces together, and the characters are very likable.
It's probably the prettiest game i have played.
Strangely enough lots of people find the lack of information at the beginning of the game a bad point, I guess they assumed you had to read the glossary in order to understand the story and never finished the game.
This is not the case, it all makes sense in the end.
So that's it, I really enjoyed this game, make sure you don't hate linearity if you're gonna try it, but other than that it's a good experience.