I've been to a lot of amusement parks, but none of them compare to Kennywood and the park is often rated as one of the top in the country.
Great amusement park food, rides (even if they haven't added anything new, you don't mess with tradition - the Noah's Ark, Grand Prix, the Turtle, the Whip!) and all around fun. Although, I believe two new rides are going to be added in 2007.
Somewhat pricey, but so is Six Flags and every other park out there...The key is to buy tickets for the local community days and have fun!