An amazingly fun and beautiful game.
The gameplay is easy to pick up and it has a learning curve of maybe five to ten minutes. The battle system is classic and it is executed very well (sometimes the battles do drag on a bit though). There are a variety of different ways to approach the different monsters that you come across on your journey, for example: Is this monster weak to Water magic? If so, I'll use Waterga on it. Is this monster flying? If so, I'll use Wakka's ranged normal attack. Depending on which enemy you are facing, you'll have to use different methods for attacking. There are "super-moves" called Overdrives that do decent amounts of damage but only once you have filled your Overdrive gauge. Aeons are summonable creatures that are quite useful in some battles as they can do a lot of damage with their Overdrives.
Instead of actually gaining individual levels in FFX you gain Sphere Levels. Sphere Levels let you move your character around the Sphere Grid and learn new abilities and magic and boost their levels. You need Spheres to activate Sphere nodes for whatever the node has (for example, Armour Break requires an Ability Sphere, a Strength node needs a Power Sphere). You get Spheres from killing monsters and from chests.
The Art style of Final Fantasy X is unique and beautiful. It's hard to explain except that it is refreshing and unusual. The graphics overall are quite good for an early PS2 game and the CG cutscenes are, as usual for Final Fantasy, amazing.
The music is excellent and quite possibly the best in a Final Fantasy game. It varies from light to intense and it does so quite splendidly.
The story is engaging and terrifically told. The characters are wonderfully voiced and developed.
There are tons of unlockables and extra bosses and minigames to do after you've played through the main story and it is definitely worth running around and spending hours doing the different minigames and looking for the numerous unlockables scattered throughout Spira.
This might just be my favourite Final Fantasy game ever because of its story, its music, and its characters. Yeah sure FFVII was good but it was really only revolutionary for its time. Don't get me wrong, FFVII is a great game, it's just not quite as good as FFX.