While there are some celebrities that can be down to earth, humble, and very nice if you do get that rare rare chance of "meeting" them at Disney World, if you were to ask them, all of them would probably express their desires to spend their vacation as a vacation meaning with family and friends, not with constant interruptions, people gawking, and people following them around everywhere. Most try to hide or blend in with the public. Snookie came to Disney World and she didn't have her signature hair. Many wear hats and sunglasses or surround themselves with lots of other random people.
I've personally seen Nick Cannon, Ryan Seacrest, Lee Dewyze, Kris Allen Mariah Carey, Penn (from Penn & Teller), Yanni, Snookie, Whoopi Goldberg, George Lucas, Joey Fatone, and just last week John Stamos. Terry Crews from Everyone Hates Chris was here last week too, but I didn't see him. I've also heard of visits from Tom Hanks, Jennifer Hudson, Miley Cyrus, the Jones Brothers, Rubben Studdard, Tiger Woods, and many more.
While sure it is exciting, you have to always remember they are people just like you an me but with a different career and lifestyle. Celebrities come to Disney all the time because of the brand and fun, but also because the way Disney treats them.