The other reviews have already covered the general idea behind this game, so I won't recap them here again. Suffice it to say that I enjoy the Tactics style of strategy RPGs, and I enjoy Pokemon, so this game was the best of both worlds for me, right up until I beat the main story.
I feel compelled to give others the warning I did not get. Once you beat the main story line, the after-game is a great many "Episodes" that can be pretty short to almost as long as the original game itself. Taken together, they make up more playtime than the main game does. Sounds great, doesn't it? There is one problem, each episode is an entirely self contained side-game. You do not get to use the team you spent the entire main game building up. You use whatever character the game says you use for that episode, and all of their link percentages are reset to whatever level the episode says it should be at. There is no New Game +, and there is no way to get back to the main "all of the castles" map once you finish the main story. It effectively takes all of the work you put into it and throws it away.
Someone would be quick to point out though that when you recruit someone in an episode, they automatically get whatever pokemon (and its evolutionary state) that they had in the main game. On the surface, this sounds like "Oh, well at least all the effort I put into the main game wasn't entirely wasted, I do at least get the pokemon back". I would like to warn you that it is more of a handicap than a help. If you are like me, you recruited every warrior you could find at the start of the game, but stopped after you got a whole team full of warlords with perfect link pokemon. This means you had/have a bunch of generic warriors with whatever weak pokemon you had at the start of the game sitting in castles filling space to make you gold by the end of the game. Why is this a problem? Many of the episodes let you fight warriors with powerful pokemon you didn't have access to in the main game. When you recruit them, the game will automatically overwrite whatever powerful pokemon they have now with whatever weak one they had in the main game. Having trouble progressing, and you see someone with a munchlax, or a deino, or a bedlam with strong attacks and lots of hitpoints that you want to recruit? Not so fast, you recruited him already in the main game, and he had a whooper there. Congratulations, your new recruit now has a whooper too instead of the good pokemon you wanted, and there is absolutely no way to prevent it from overwriting him. You also cannot access your main game anymore to try and stock them up with more useable pokemon to use in the episodes. You are forever locked with whatever junk you left them rotting in the castle with.
The game really is fun. And I am sure there will be people that love the after-game episodes, but I am not one of them. I feel like I put time and effort into building my team and their link percentages only to have it basically erased without my consent. The carrot for those episodes is to recruit new warlords and get legendaries for the ones you had, like Mewtwo and Groudon. The only problem is that once you get those legendaries, the episode ends, and you don't get to use them again. What is the point of having all of these legendaries and warlord transformations when the chances are you will never actually get to use any of them?