While there's plenty of Monsters to capture and breed, a lack of depth and buggy controls could frustrate a few people.
Summary: You live in a world of two kinds of people: Those that can see Monsters, called Neo-Seeds, and those that can't. You discover that you are a Neo-Seed, and go on a quest to defeat what I'll just call Boss Monsters. There are 9 types of Monsters. Animal, Plant, Fire, Water, Rock, Wind, Light, Dark, and Spirit. As you go on questing, you'll run into monsters and fight them. Whether you catch them or obliterate them is up to you.
Learning the how everything works is clear and simple. It's implementing it that's hard. Fights are more interactive then Pokemon. You press the right direction on the D-Pad to guard from an attack an receive less damage. Time your attacks right to perform a chain and deal extra damage. Everything is relatively easy, except for catching a monster. To catch a Monster, you must press A as fast as you can. The faster you press, the more likely the monster will be captured. Sounds easy? Wrong. The first Monster I captured, the percentage was 7%, and I'm a good button-masher. Luckily, your first monster encounter is guranteed capture. Not so after that. I'm a mid-teen, and I found capturing difficult. If your younger, or would prefer to make it easier, The capture difficulty is set by how fast you beat up a person near the beginning of the game. I'm not going to give any spoilers, but when your trying to get something back from a friend, press A more slowly then you usually would. Trust me, It makes the captures about 3x easier.
Now the Monsters. There are over 160 monsters to capture and breed. In theory, this would keep you busy for quite a while if you're into getting them all. Sadly, much of the fun is taken away by the evolutions. When you "evolve" a monster, they don't change shape. They change colors. And sometimes not even that is noticeable. Not even the names change. Here's an example.
Monster-->Monster Plus Monster Plus-->Mega Monster
This example applies to all monsters that can evolve.
So in reality, There's roughly 55 different monsters to capture. Don't get me wrong, I still get blast from finding them. Some you can only get by mating. And they can look pretty sweet.
For capturing monsters, instead of Pokeballs, you use Essence Cores.(EC) You can only catch a Monster with an EC of it's own type. For example: Fire-Monster, use Fire EC. Many times I have found myself with some awesome looking beast in front of me, and nothing to catch it with.
Since I can't give any spoilers, I really can't rate the story line with anything but a number. So... 6/10. But hey, who ever plays these game because of the story?
Overall, despite some of the negative things I've typed in, there's something about this game that makes you want to keep playing. Whether it's completing the storyline or the side quests or breeding to get "Supreme" monsters, I keep on pulling it out time to time. So I give it an 8.