I've had Mastiff's since 1990; we currently have two; they are wonderful, wonderful dogs! Although I'm not that pleased with our older dog who ate a stick this AM and just threw it up on the floor.
As has been pointed out, they do slobber, but primarily after eating and drinking, ours are house dogs, but they do all eating and drinking, other than snacks, outside. Once you have decided on an OEM, I would research the breeder, my primary concern has always been health and temperment, it's a good sign that your breeder does testing. The only other downside to the breed is their relatively short lifespan, generally 8-10 years, but that is true with any large breed dog. I don't think that eat all that much for animals their size, our dogs both have birthdays in April, our older dog turned three and the younger one turned one last April, about 210 lbs and 150 lbs respectively, anyway, they seem to go through about 80 lbs of food/month. They are really just big, big babies, but can be very protective if they feel their turf or master threatened......and when they get wound up it is an impressive site!
This pic was taken last year around Christmas