Castle Crashers kind of put me off with its high price tag at first but when I finally gave in I was glad.
You can play this game alone and it's crazy fun but it is so much better when you jump into the 4 player coop. There are a good amount of levels to go through and a ton of collectibles/unlockables. There are tons of weapons and pets to find throughout the game and a ton of playable characters to unlock as well. For the collectors out there, this game will give you a lot of bang for you buck. There are a ton of laughs through this game. As the plot unfolds you will find yourself laughing more and more from movie references, the slap stick humor and other things.
There are a few other multiplayer modes that add a little extra to the game but you won't find yourself actually playing that much at all. I won't even bother to mention them because all you really need is the single player/coop.
For you achievement junkies out there, this game is a simple 200/200 until you have to get the 2 online PVP achievements. I think you will only be able to get those achievements boosting or spending a LOT of time trying to get. Your fingers will be aching after those 2.
This game is worth the high price and you will get a lot out of it. If you have 1200MS to spend, this game is a great choice.