The day you pick them up from the shelter and the second day when they're still on their best behavior. After that they're useless. Or is that just my experience picking them?
Dogs need time to adjust. Spending time with them, training them, and lots of positive reinforcement is the key. They need to learn to trust their human.
I love my dog so much I know my heart will be broken when she's gone. But also she is so deviously smart that I want to throttle her for the things she gets up to. And we love them anyway.