It's a typical action-RPG, but a huge array of side quests and a deep party recruitment system make it special.
Most of these side quests revolve around recruiting other characters to join your party. These characters, over 175 of them, all have their own stories and personalities that will keep you interested; collecting them is extremely addictive. The method of recruiting characters varies for each one: sometimes you'll simply have to defeat them in battle (you can challenge almost any character in the game to a fight by kicking them in the shins a few times), while other quests are more complicated, and many involve the real-time scheduling system that governs each character's actions throughout the day. You can spend hours just wandering around the city and following people as they go through their routines.
Other side-quests are initiated after joining Theater Vancoor, an organization of mercenaries that will pay you to do quests and miscellaneous errands for their clients. There's quite a lot of engaging gameplay here, and you can make good money in the process.
As I said, the main story is nothing special, and this applies to the writing as well. People often mention the humor of the game, but if your sense of humor is like mine, you'll be laughing harder at the cheesy melodrama than the awful jokes (the grating voice acting doesn't help). Now, I actually like RPGs to have laughable stories, but if you're looking for an inspired narrative, you may be disappointed.
The game's world is quite enormous. You'll be spending most of your time in the main city, but there are plenty of varied towns and dungeons outside the walls of Radiata. You can enter most areas at once upon starting the game, but you won't be able to get very far due to the extreme difficulty of the monsters in some areas. You'll have to do a bit of grinding in other places if you want to survive.
The visual style of Radiata and its environs is warm, colorful and cartoonish, and will often remind you of World of Warcraft. The character designs are anime-like but are more exaggerated than most characters in Final Fantasy-style RPGs.
The bottom line: if you like deep exploration and freedom in RPGs, and are willing to put up with a cliched story, then buy this game immediately. You'll be engrossed for weeks.