I don't know about a site that is dedicated to tobacco's, but in regards to "pipe tobaccos,"
www.tobaccoreviews.com is a great web page.
You will probably want to hit smokingspipes.com, and other vendors who sell pipe tobacco, as their customers will post reviews on the page that features a particular pipe tobacco you are considering to purchase. For example:
http://www.smokingpipes.com/tobacco/...product_id=146