I purchased a Atomik Hummer 150cc for my 14 year old son from e-bay, and I couldn't be happier, Sure you get what you pay for to tell you the truth I am not sure how they can produce them for the money they are selling them.
Sure they are not going to last as long as a brand name bike, if your not happy with the quality go out and spend 7-8 times the money and purchase a brand name bike.
Before all you brand name bike owners flame me, I am on your side I own a Honda XR200R ( which the Hummer can give it a run for it's money although my son is a bit more fearless when it comes to racing than me my 44 year old body hurts to much when I fall off ) and sure things tend to not break as much.
I can tell you if it wasn't for the cheap China bikes there is a very good chance that my son wouldn't be riding a bike at all as I can't afford to purchase a Honda,Yamaha for a kid that will grow out of it within a couple of years, I am sure there are guys/gals out there that will insist on buying name brand bikes and don't care about the price and that is great but don't flame people that only have the finacial means to purchase a cheap bike that there kids can get a great deal of fun out of, my son is just glad he has a bike to ride.
Sorry for rambling I will go now.