In the grand scheme of Japanese Role Playing Games (JRPGs), Final Fantasy XIII is a good game. Many people absolutely love FFXIII and many others strongly dislike it with few in-between opinions. On Metacritic, FFXIII is one of the more poorly reviewed games in the series. (metacritic of 63 for the PC re-release and 83 for the original PS3 release, while the rest of the main entry games tend to get 90+ scores on metacritic... FFXII - 92, FFVII - 92, FFX - 92, FFIX - 94)
Personally I was very disappointed in Final Fantasy XIII. It just didn't feel like a Final Fantasy game to me. The classics of the series for me personally are IV, VI, VII, IX, and X. The sequels to Final Fantasy XIII are also heavily divided amongst fans. While I strongly disliked FFXIII, I actually quite enjoyed FFXIII-2 and Lightning Returns. So, everyone's individual preferences will vary and I always suggest gamers to give the games a try themselves if they are interested in the game.