Haven't been, but we are going this year with kids who will be 4 1/2 and 7. My personal opinion, the 4 year old is a little young. But, I don't want to make the 7 year wait any longer. I think she is at the perfect age - but that age doesn't last long.
At a certain age, and as I remember for myself this was about 8, kids develop a much more realistic perspective on life. A little of the
ability to experience things in that magic way that little kids do, disappears. Think Santa Claus.
My daughter knows what Disney is (thanks to TV and friends) and is old enough to appreciate what a big deal this trip is. She can anticipate and plan, and thats half the fun. She understands its a one time thing too. And she's still young enough (I hope) to see Disney through a kid's eyes - where I might meet a man in a mouse suit, she'll meet Mickey Mouse. At least that's what I hope.
My son...well. He has an idea of what Disney is, but he really doesn't understand what a big deal it is. Its just some place we are going. Out of the blue, he has, on different occassions, informed me that he's scared of airplanes (this was a casual revelation) , and that he doesn't want to go to Disney because its too hot there (that would be Grandma talking). He just doesn't entirely GET it.
So, I would say 5 3/4 is not "the" ideal age - if you really want the maximum first-and-only time Disney experience bang. Another year of maturity will allow the child to more fully appreciate the trip. But, if you are going to take him regularily, then might as well go this year. We've been taking our kids to local zoos, safaries, theme parks etc since they were babies, and have never had any problems sticking out a sunrise-sunset day in the parks. 5 year olds certainly like this stuff, they just might not completely appreciate what a big deal it is