They have lots of health problems, and when I had my dane mix he ate about 100lbs of food a week. They are prone to bloat, and shouldnt be fed right before excercise, and should be trained from an early age since their size can do damage.
I have owned 6 in my lifetime, 3 had been rescues, 1 I purchased and the other 2 were gave to me. Each had their own individual personality but all in all they were mild tempered with the exception of one female she had not been spayed when I got her. When she had her first/only heat cycle with me she became a killing machine anything on 4 legs was fair game. Needless to say she attacked and killed a dog, the owners insisted she be put down however the vet made a few phone calls and we placed her in a country home about 60 miles from where we lived..the owners of the dog she killed never knew we didn't put her down. They had her spayed and kept in contact with me throughout the next couple of years on how she was doing( lived in the lap of luxury) lol anyhow she was the only one with temperament issues the others were oversized lap dogs that loved everyone/everything.