I read these as a child and enjoyed them, but I knew that I would never have the gumption to do the Great Brain’s schemes. Not that I wasn’t brave enough or clever enough, but that I never had the taste for deceiving my friends and family. He cared more about money and power than he did about anything, and he routinely blackmailed and manipulated his younger siblings. You could admire him because of his cleverness, but he was not a good person; you knew at the end of the day he would sell anyone down the river. Being good at making money doesn’t make you a decent human being. Being really experienced at deceiving people precludes being a decent human being.
At least, that was my take reading them as a child. Maybe it would be different if I reread them today.