While the changes will turn off loyal fans, I found them to do more good than bad, making a very enjoyable experience.
The bad of the game is in its weak story, which is my least favorite in the series but, this is not to say its complete crap, it just wasn't as big like the stories of VI or VIII. And although most people will say that Vanille is a negative aspect of the game, I actually found most of the characters (with the exception of Lightning and Fang) were whiny and annoying but i can forgive Hope somewhat as he makes up for it by being awesomely powerful once u get him going. The final negative of the game is the crystarium system. I was never a fan of most of the systems in Final Fantasy, but I usually get used to them and although this happened in XIII as well, the part before I got used to it was rather frustrating.
All in all, Final Fantasy is still one of my favorite franchises and most fans will love the game, even with its somewhat dramatic changes.