If you have the money for the sigma 100-300mm f4 and have a jessops near you with stock, then get it, at £600, nearly 1/2 price because jessops are no longer stocking sigma lenses I would get it, If you don't like it, your still quids in because new everywhere else its retailing at £1150, so you'll easily recoup money and some. A no brainer really. Its a very good lens, especially at £600, you won't be disappointed.
Peter