If you think someone is nice you can him or her a good egg, if they aren't, then they are a bad egg. If that bad egg is a bad influence on others, than that person is called a rotten (pourri) egg because he or she is good for nothing.
Some people are just too smart for their own good and are so "intellectual" that they are out of touch with reality. These are often very smart but lacking all common sense. We call these people, particularly in American English, eggheads. Often, these same eggheads are stubborn and won't listen reason, if that's the case, then we can call them a hardboiled egg.
Others of us aren't so intellectual and we aren't good at organizing or thinking straight. If you are of these type of people, you have scrambled eggs for brains.
Well, I should stop here because I don't want to be eggy (embêtant) and egg you on (embêter) further!