Different between madly in love and in love, I do believe they both means the same, When someone says "I love you", love is a very strong word, and Nobody really know how much love is enough, They do feel it in their heart, but can anyone describe "love"? Maybe certain amount of people can, If they seek through their heart, and feel the differences between the time you loved your ex, and the time you love your current one, You could tell the differences. Love isn't something you say just to make people happy, it's something that is truly from the heart.
And for, madly in love, is when they are too crazy for one another, unconditional love which takes patience and understanding, when they get into an argument. Madly in love can also mean you both are crazy for one another, cannot stop thinking about each others, means the whole world to each others. That kind of love takes control of you.
I've heard about some people say, I love my wife/husband/girlfriend/boyfriend so much with all my heart, but then they go out cheating on them? Now that's not love. Love is a powerful word, and it has to meant it from the heart. You don't say "I love you" when you don't mean it.