We had a Bullmastiff for 9 years. You would have to look hard for a more loveable and gentle animal.
We have a picture from when my son was a toddler of Beckett lying in front of the wood heater with my son sleeping on his back and the siamese cat curled up between his legs.
Be aware though that they are LARGE! Training is a must since the average person is not physically able to hold them if they decide they are going somewhere. Also, ours chewed doorframes and snored (really loud!). We took him to the lake and he loved the water, but he had the bouyancy of a block of granite (he walked around on the bottom even if it was over his head). If I chased the kids, he would clamp on to my thigh (didn't really hurt, but you try to run with a 130lb dog hanging from your leg!).
I took him to a Renaissance Fair one year and everyone there fussed over him. I teased my wife that I could have gone home with any number of good looking women because they would have put up with me just to get the dog.
Unfortunately, bullmastiffs have some health issues. We had to have surgery done on his eyelids (they turned inward and irritated his eyes) and both hips (dysplasia).
He was hit by a pickup truck once, but showed no indication of pain so we didn't know it until our neighbor called and said "I just hit your dog with my truck". We took him to the vet and found that his leg was broken in 5 places (heeled fine though).
A big problem with large dogs is that they simply don't seem to live as long as the smaller breeds. The day we had to have him put to sleep (cancer of the spine), I was at work; I locked myself in my office and bawled for two hours. God, I loved that dog!