Sunset Magazine ran a feature article on it in their last issue that stressed how lean bison meat is supposed to be. But I looked it up on Calorieking and 1 oz of ground bison is 63 calories with 4.5g fat (2 of them saturated).
That's definitely NOT leaner than the ground beef I get, which is 38 calories per ounce with just over 1g of fat (and only .4 of it saturated). The bison meat is close to twice as fatty as my ground beef, which pretty much takes bison meat off the table for me.