Not bad, but...
The idea is really basic and nothing's really going on. The animations weren't smooth at all, in fact, some of them (not all, just some) were bad. The were pretty boring, there could've been some more creativity in textures or different colors (colored cardboard paper?). What I'm trying to get at is: it's boring. Don't get me wrong, you all have talent, but you definitely could've added some more work to it.
If I lived in a city made of cardboard, I'd walk around with a radio pumping jazz like, ALL THE TIME. just imagine how awesome that would be.
myself I thought the animations were quite smooth, but hey, different tastes, different standards, its all good. :)
Cartoon?
Newgrounds is meant for cartoons, not full movies and films. I think you should re-read the Newgrounds ToS since it does says that a submission MUST be a drawn cartoon. Please, if you want everyone to see your movie so bad, submit it at YouTube.
Age 32, Male
Joined on 7/14/06
Cardboard City 
Four Minutes (part 1)