I had a Great Dane for 10 1/2 years. She was an awesome dog, very well mannered, sweet & gentle. As they say: Great Danes are Gentle Giants. I took her to obedience classes and she did extremely well, walked on her leash just fine, loved every dog, cat even a fox that walked slowly past her, wanted to play with him, ( rear end up ) Too cute! She did have bloat which was corrected as soon as we noticed the symptoms, took her the emergency at our Vet. After that she lived another 6 years until the same occurred again. Sadly we had to send her to " doggie heaven ". To sum it up, this breed is wonderful with kids, other animals in your house and would even invite a Robber and show him where the silver is hidden. Get it ? Ha ha
My mother has one. It's a sweet dog and not aggressive at all. The only thing is it is huge. It takes any kind of food or just whatever she is interested in off any table in the house. That's pretty much the only drawback besides health issues. Apparently, they need to have their stomach stapled to their ribcage to keep it from flipping around inside of them. They may have other health issues, too, like all purebred dogs do.
Great Dane are good pets but it only matters if its great with your family like if you have little siblings or child... But to be sure do some background checks on the breeds you like and in animal kingdom they have that one show that gives you info about breeds and its called "DOGS 101"