Kingdom Hearts 358/2 days was definitely worth its hype. If you're a fan of the series, it's definitely worth a buy.
The panel system makes a nice amount of customization to your abilities, working in shapes and sizes, rather than just the numeric values from the other games. The system works well, alongside the magic shift, and it just makes management another fun feature of the game.
Challenges and Mission mode add tons of replay-ability to the game. Almost every mission has a challenge, and a mission mode version adding hours to the possible play time, even after the main storyline is finished. Each Character in mission mode has a different play style, making the missions slightly different depending on who you play it with.
The battle system works amazingly, allowing you to easily switch between targets, and cast off a magic spell at swarms of heartless mid-fight. All of the character's combos work out seamlessly, with no execution problems.
Overall, Kingdom Hearts 358/2 is a great game. I definitely recommend it to most gamers out there. The only real problem is that to fully understand the game, you must have played its prequels, Kingdom Hearts (the original) and Kingdom Hearts : Chain of Memories.