The flip side-
My neighbor has a Mastiff as well-her 3rd actually, sadly this breed has a life span of 7-9 yr. tops. Frankly, most English Mastiffs are "too much dog" for the average household. Very few people really have the knowledge, facilities, or skills necessary to manage this breed. In addition, there is shedding
snorting, snuffling, wheezing, grunting, loud snoring, lots of slobbering and drooling oh, and lets not forget-lots of gassiness.
I visit my neighbor fairly often-I've never had a problem w/her dog, but unfortunately I can't bring my dogs (a Dobe and 2 Shepherds) over for a play date. Some Mastiffs will not tolerate another dog of the same sex, and some won't tolerate the opposite sex either.
Another issue-The Mastiff may be targeted for "banning" in certain areas, or refusal of homeowner insurance policies. In this day and age, the legal liabilities of owning any breed that looks intimidating and has a history as a guard dog should be seriously considered. People are quicker to sue if such a dog does anything even remotely questionable.
From the English Mastiffs website:
Mastiffs are not Golden Retrievers. They have an independent mind of their own and are not pushovers to raise and train. Some English Mastiffs are willful, obstinate, and dominant (they want to be the boss) and will make you prove that you can make them do things. You must show them, through absolute consistency, that you mean what you say.
Like cattitude posted-do a lot of research! Good luck w/your quest..