Bullmastiffs are protective. They can play with other dogs and often do, but they MUST be socialized from a young age with other well-adjusted and well-socialized dogs. Try to keep them from unstable or poorly trained dogs. They tend to lie around as adults but will do most anything, other than a lot of strenuous exercise or any exercise in the heat - they are not heat tolerant. My BMs all sank like stones in the water but maybe some swim! They are wonderful with children but MUST be trained and socialized from an early age. They do not need much exercise, but they do need some. Some are lap dogs, some are clowns, some are aloof - you never know. They are not easy to train, as a rule, due to their natural general temperament related to the reason they were developed originally.
I have to emphasize training and socialization, and can't really overstate this. In the absence of good leadership and direction they will want to take over and be the boss, and you don't want that from a huge, powerful dog. Even under the best of circumstances they will attempt to take over during the teenage years, and this is where continued training and socialization will benefit everyone. They are bred to sit in the dark and watch and wait, so when a BM is "lying around" and sleeping all the time, they are actually working. They are bred to make their own decisions about when to act, and that means they are independent thinkers, not people-pleasers (in general). If you train them properly and show them what you want from them in terms of acceptable/desirable/unacceptable behaviors then you can let them do what they do, and trust their reactions. None of my BMs ever barked at the door or made a fuss over company, UNLESS the company was not to be trusted, and then my BM was standing bodily between me and the other person, or they'd block the door to prevent entry. They aren't "attack" dogs but you can believe they will protect you. I don't know that they protect PROPERTY, per se, in they'll protect YOU. It is awesome and a great feeling. You don't have to tell them what to do - they already know.
They tend to have food allergies (chicken and corn, commonly) and are EXPENSIVE dogs to own. Everything at the vet costs a lot more due to their size, and they can be prone to health problems, unfortunately. They slobber (some not as bad as others) and this will get slung all over the house. They shed a lot. Their nails are large and thick and it is not a simple thing keeping them short. Get them used to this procedure at a young age. There is a lot of information here about spaying/neutering too that is important as well.
There are a lot of exceptions to what I wrote in the first paragraph. Sometimes male bullmastiffs cannot be in the presence of another male dog of any breed, and sometimes its okay, depending on their training/socialization/temperament. These are not dogs that I would recommend to anyone just wanting an all around easy-going family dog. This is a dog that could be a danger in the wrong hands or just raised without leadership. Poor breeding can create dogs with horrible temperament that can't be corrected with training, so look for a reputable breeder - I could write 700 paragraphs about that, lol, so search here for info about signs of a reputable breeder.
Good luck in your search, and there is a ton of great information here. Browse through the forum, including health/nutrition and training/behavior to get an idea of what kinds of things you might expect. I would have 12 of them if I could. Today.
EDIT: Ditto to Denna's Mom's comment about the reaction of those around you. Your family, friends and neighbors may be quite afraid of your dog, and you will for sure get a strong reaction out in public. In my dogs' cases, other dogs often reacted strongly to my dog, with their large and imposing presence, with a huge head and dark face. It is not anything like having a large dog of another breed that is not as tough looking.
