We absolutely loved 1900 Park Fare for breakfast on our 2006 trip. We were so disappointed when it was closed for refurbishment this summer, because we were looking forward to it again.
Foodwise, of all the Disney buffets we've tried, I think this was my favorite. I loved the variety. Hard to explain, but to me it just seemed a little more "posh" than some of the other character buffets.
We had the earliest reservation they would give, which I think was 8:00. When we got there, Mary Poppins wasn't there yet. (We were the first ones to the podium, and were literally seated at our table within a minute of them opening the front door.) Mary Poppins normally sits on a couch in the waiting area of the restaurant and groups get to meet her one at a time as they're going in. We did get to meet her and have our picture taken with her on our way out, though.
I know they always give you the disclaimer that specific characters may not appear, and I guess any CM is subject to an occassional sick day. But as it's advertised as the "practically perfect in every way" breakfast with Mary Poppins and Pals, it's probably just a very slight chance that she won't be there.
I think the characters inside who come around to the tables are the ones that fluxuate more often. When we were there, it was tigger, Alice, and the Mad Hatter.