I think it's interesting when an author enters another fictional character in the world of the famous and elite. For example, I'm reading the Wood Wife right now, and one of the characters is a famous poet who was friends with real famous poets, like Neruda. It's an interesting concept.

I also don't mind it when an author chooses to make the main character of a book a famous person, but it probably helps when that person isn't alive anymore. Well-researched famous people in books is okay, but poorly researched is distracting. There's a good way and a bad way to do it. That is how I feel. The end :)