I know this thread is long dead but I just wanted to add my experiences for anyone who might find this in a search like I did. I rescued a German Shepherd/Great Dane cross eight months ago. I love her dearly, but she is a handful and I imagine many less-than-passionate dog owners would have given up on her by now. She was only six months old when I got her, but she had already been adopted and returned for being "destructive". These are a few of the issues I've had with her:

-She has severe separation anxiety. When I leave the house, I have to lock her in a crate and secure the door with around 5 or 6 carabiner hooks to keep her in. The hooks get replaced often as she can sometimes break a few of them in the time I'm away for work, especially if it rains as she has a bit of storm anxiety as well. If she manages to escape and finds the house empty, she will tear the molding off the walls and chew on the furniture. If we are shorthanded a work and I need to put in overtime, she becomes so distressed by the decreased time together that she acts neurotic and is nearly unmanageable.

-She is extremely timid. As far as I know she's never been abused, yet if she is spoken to in a harsh or angry voice she will panic and thrash around like a spooked horse. I've ended up with scratches on my face and neck from her doing this.

-She does not learn commands easily. So far the only thing she will do reliably is sit, and sometimes she will lay down or roll over. She is not completely housebroken yet.

-I am beginning to suspect she may have sebborhea. I bathe her every Monday and by Thursday she is as dirty and smelly as my old roommate's Dalmatian mix who was only bathed once or twice a year.

Don't misunderstand me, I love my dog and I am totally committed to working with her through any issues she has now or may develop in the future. But I know most people just want a dog to be a companion and are not willing to make a lifestyle of it, and for that reason I STRONGLY discourage anyone from creating more GSD/GD mixes. You may get lucky and end up with a litter of well-behaved, even-tempered pups, or you may end up with a whole bunch of near-impossible-to-adopt-out 100lb. maniacs. I see that others before me have already covered the overpopulation/euthanasia angle, so I won't go too far into that except to say that ANYONE who considers breeding ANY dog for ANY reason should visit a shelter in a high-kill community first. If you have the resources to care for a litter of puppies and are willing to deal with the unpredictablity of mixed breeds, please consider fostering a few for your local shelter or rescue group!