I've played the two mainline games and the CoM remake on PS2. If you're a Disney or Square fan (or better yet, a fan of both) and want to get those nostalgic warm pangs in your heart, then this is the series for you. The story is extremely convoluted and no one really knows what the hell is going on, but KH isn't really about that imho. It's about being sappy, nostalgic, and plucking at your heartstrings. If those things don't sound appealing to you, then stay far away from this series.
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Yoblazer: http://i32.tinypic.com/zlf1w7.gif
Watch and you'll see... someday I'll be... part of your world!