The story is probably one of the worst Metal Gear stories ever and I absolutely hate what they did to Raiden's character. I also am not a fan of how this game essentially rehashes themes from MGS2 and makes everything about MGS4's ending completely meaningless. The new characters are very loveable though.
Bad story aside, it's a really fun action game. I wish it had actual weapon switching though -- I always end up just sticking to the Katana and not bothering to use other weapons because there's just no reason to.