I do empathize with complaints about the noise factor. But here was our situatuation: Our 7 yr old son began collecting Zhu Zhus when he was six. This was about the time he wanted a dog, and I was not ready to take on the responsibility (caregiving for my mom with cancer at the time) and this toy really fullfilled his need to nurture something. And I didn't have to worry about cleaning up poop :-)
My son is gifted, so his interests tend to lean towards more complex, imaginary play. He's also very sensitive and nurturing. He was more interested in the unique noises each Zhu Zhu makes - their own personal language, is how he thought of it - rather than any movement they make. He took a great deal of pride in 'nurturing' each pet, and still likes to take one with him on playdates or for show and tell at school. He also liked their unique looks. He was far less interested in the accessories that came with the pets, except some of the outfits. So, personally, I wouldn't bother with accessories until the child shows more interest in what the pet can DO than the pet itself.
Some kids particularly like the hamster wheel that connects to the tunnel, because each pet makes it's own noise when traveling the hamster wheel.
As for cost, my son 'earned' his pets via chores he did at home or while at his grandmother's. We found most of them for around $6 at CVS or Walgreens. When they first came out they were selling for around $20+ each. :-S
There is an off-brand version off Zhu-Zhu pets in Target (I forget the name) and they are cheaper, and less noisy. My son started collecting some of those too, but I don't think he likes them as much.
Hope this helps :-)