I've thought about getting a Matiff, I like Newfs, but I've always thought a Bull or English Matiff would be cool too. I've read that they have very good temperments, certainly if the dog (matiff) was a killer we would be reading about it instead of pit's and rot's.
You need to be careful of the breeder. Bull Mastiffs are lovely dogs, not as large as OEMs, but plenty big enough...
Sometimes a mastiff will be shy, or aggressive...such a dog should be put down and never ever bred, but as with anything, some unscrupulous people will, for money...shy dogs are as dangerous as aggressive ones, imho.