Opinions on German Shepard's

[B.Warning]

Starting to consider getting another dog. I have 8yr old boxer now and has been a great dog. I couldn't ask for a better companion than what he has been. Loyal, great with kids, good watch dog, but just a dumb breed for lack of better words. My grandfather use to tell me about his German Shepard he use to have and how great of a dog he was so that got me thinking about getting one.

Couple questions....

Are they a easily trainable dog, or do you have to constantly work with them?

Are they guard dogs by nature, or trained to be?

Good with kids and infants crawling over them?

Smart, obedient?

Pretty much I want a dog that can jump in the bed of truck and take off. I'm done with having to have my boxer in the cab with me non stop, he will not stay in the bed and has jumped out at highway speeds before. I also want a dog that I can tell to stay, leave the room, and return with him in the same spot I told him to be. Again...no matter how much I worked with my boxer he simply is not the smartest dog.

Thanks for the input.