I'm not going to say a whole lot but this is what I found out on the net regarding Heath's performance: "With all due respect to the enjoyable camp buffoonery of past Jokers like Cesar Romero and Jack Nicholson, Ledger makes them look like ... well, clowns."
The quote above was taken from MSN @ http://movies.msn.com/movies/movie-critic-reviews/the-dark-knight/ if your interested in reading the entire review.
I loved Jack's performance & I thought it might be hard for anyone else to even step in his shoes. On a personal note, I havn't seen the move yet but I have pretty high expectations now!!!
Enjoy