I adopted my Gizmo when he was around one (or so I was told I think my old soul pup is older than I'm led to believe) and he was in very rough shape. It was horrifying. He was sick sick and it took about 4 month for me to nurse him back to health with much medication 3 times a day, patience and love...
That didn't include some of the major behavioural issues he was facing as a rescue. He didn't want to walk, ever. He hated the grass. He was afraid of EVERYTHING, massive separation anxiety, he couldn't even eat properly when he first arrived and needed serious dental surgery. He quickly became Mummies little guy though. However, we noticed him displaying some amazing improvements when with my best friend's yorkie...things he never did on his own!!
We decided to add Luna in to our family and from my perspective it has been the best thing that has happened for us. That being said, I am eXHAUSTED. I love Luna but she is a puppy and puppies are hard work. She had me up every two hours to go out to pee for the first couple of months. She has so much energy. BUT - she has taught Gizmo how to be a dog, something I could never do. Sure, she's the alpha and he is sometimes not it the mood for puppy all the time. But he plays now. It melts my heart. We go on great walks. He plays fetch and will chew toys...Things that for months held no interest for him. Every day as Luna gets older she becomes more obedient and fits into our routine more and more...
I guess my personal opinion is that it's a great addition to the family -- as long as you're prepared for the extra work
Gizmo is smiling at me as I type this, he's going to take a pass from Luna's 530 am walk tomorrow morning
Best of luck.