Readers of Patricia Cornwell's Jack the Ripper -- Case Closed should find this counterpoint site of interest. While the authors of these articles engage in almost as much cherry-picking refuting Cornwell as she did making her case, they still provide an interesting counter-argument to the book.

I read the book over the summer, and while I found it interesting (at least as much for her vivid depiction of late 19th-century London as anything else), I found her conclusions interesting but largely circumstantial. Still, what she's found implicating Sickert seems to be more than anyone else has found for any other suspect, so for me Sickert is certainly a suspect, albeit not a "definitive" one.