Answer: Pure heaven, if heaven tasted like melty, cheesy goodness. Which it should.
Technically, a croque monsieur is as low-brow as French cuisine can get. The French wouldn't even call it cuisine. To them, it's like what peanut butter and jelly sandwiches are to Americans: basic, plain, and not something served in "real" restaurants. But to me, it's like crack, except a horrible kind of crack that makes you gain weight instead of lose weight. This wouldn't be so bad, except for the fact that I want one all the time. (See my entry entitled "Beware of French Weight.")
Croque monsieurs are open faced sandwiches of ham, Gruyère cheese, and French bread with a light layer of Dijon mustard, and a béchamel sauce (comprised of salt, pepper, flour, nutmeg, and butter), which is then baked in the oven until it culminates into an explosion of magical flavor that would make you cry, it is so good.
It's probably a good thing that the French don't believe in drunk-eating, and letting restaurants stay open 24 hours, so that alcohol-fueled croque monsieur binge sessions are out of the question, or I would really fulfill the stereotype of the obese American.