When my son wanted to get a mastiff I told him he couldn't bring a mean dog over around my two small dogs. He told me that I wouldn't believe how gentle Mastiffs are and he was right.
This dog is the perfect companion and yes he does like to sit on your lap and lay on the couch with you. At first the gas and slobber were a bit off putting, but they learned if they fed him quality food he didn't have those problems as much.
He is spoiled by me, he has his own stuffed animals in a basket at my house and the first thing he does when he comes over is get every toy out! He then brings them to me one at a time and we play tug with them for hours. He can almost pull your arm off with jerks of his head! Then he sleeps......and sleeps......and snores......on my feet.
They got him at 12 weeks old. Here is his first day in his new home:

As far as walking goes, he never pulls you because my son has taught him well. When he is with me on walks he is not on a leash because we are out on a farm and he doesn't need a leash. He responds to all commands but I have to lower my voice to sound more like my son to get him to respond quickly. He ran into the horses pasture one day and she chased him out, that is the only time he has gotten near her, he is a quick learner.
This dog is all love and fun. I would recommend a Mastiff to anyone, especially a family with kids. When kids are around him he intentionally moves carefully so he doesn't knock them down. It's a stitch to watch!
Thanks for the responses!