I agree with them not growing up and stuff-- BUT here's what I say:

Nintendogs is good for people who don't have a dog
Nintendogs lets you have choices
You can walk your dog, train your dog, and put your dogs in contests
It can be repetitive, but there's always new things! Like save for the space room or connect with new people!
You can buy new dogs and get new items
You can teach them by voice, and pet them with your finger-- very realistic

But other then those, I agree...At first, it's awesome: "OMGOSH DIS IS SOOO KEWL!" But then after a month or two, it grows off. So, yeah; it can get boring