My grandpa had Alzheimer's, and I think he genuinely enjoyed the early stages of it. He was a simple, carefree kind of guy who was happiest just sitting on a lawn chair with a beer in his hand, and my grandma was an accountant who kept meticulous track of everything so he didn't have to. For him, ignorance was bliss.
But the latter stages are just awful. He spent all day, every day looking confused and upset. He didn't recognize friends and family. Lifelong Cubs fan but he couldn't watch anymore because he couldn't remember how that guy got on 2nd base, much less who he was or how the team was doing. He couldn't even recall how to play the solitaire variations he had used to pass the time his whole life.
It's just sitting alone, confused and miserable, with no ability to grasp what the problems are, much less how you might change them, all day every day until you're lucky enough to die. If I ever get diagnosed with Alzheimer's, I intend to buy a gun to speed along that process.