I love a bargain as much as the next guy so when I was looking for dirtbikes for my kids, I was an easy sell for the Chinese bikes.  I bought 3 different bikes from 3 different manufacturers:  a 50cc 2-stroke on eBay for $235, a 70cc 4-stroke for $600 and a 90cc 4-stroke for $800 from local "dealers".  In addition, my best friend bought a Chinese dune-buggy style go-kart last year and another friend has a Chinese 125cc dirtbike.  I feel like I've had a broad experience with the Chinese bikes...so like Hans & Franz used to say on Saturday Night Live..."Hear me now and believe me later!" 

In short, they're not worth the money.  My recommendation is to buy a used Japanese bike (Honda, Kawasaki, Suzuki or Yamaha).  You'll spend approximately the same money and will enjoy them more.  I really thought I was getting a great deal but what I got was a great deal of trouble.  After I finally "wised up" I bought a used Honda XR50, Kawasaki KX65 & Kawasaki KX125.  The maintenance is actually less on these bikes than it was on the Chinese bikes.

Here's some of the troubles I experienced:  First, there is very little resale value on these bikes and they're difficult to get rid of.  The 4 stroke engines are weak and underpowered.  Don't fool yourself into thinking that a Chinese 70cc can keep up with a Honda 70cc; you'll eat dust quickly and get lapped early and often.  We were riding with my friend who has the Chinese 125 and he asked to ride our KX125 for a few minutes.  After he returned, he put his bike on the trailer and didn't ride again the rest of the day because he was so bummed about his bike.

The shifters are difficult to operate, especially for kids.  It's not the shape or adjustability of the lever, it's just very stiff and hard to move.  They also have a strange shift pattern which is opposite of normal motorcycles.  My son still gets confused sometimes when shifting his KX65 because of learning on the Chinese bike. 

The metals used (both steel & aluminum) is not of good quality and breaks or cracks easily.  My friend's go-kart front wheel mount broke within 20 minutes of riding.  He took it back and they exchanged it for another kart but the rear axle of that one broke a month later.  Last week after 30 minutes of riding it the torque converter belt broke.  No one in the "small town" of Atlanta has one to fit; it must be ordered from China.  This kart is at his lakehouse and only gets ridden a couple of hours every month or two so it certainly should not have these types of problems.  My 9 year old broke the rear shock of the 70cc while riding in a field 2 months after he got it...he wasn't jumping, it just ripped apart on very small bumps.

Many manufacturers claim to have Honda-cloned engines but although they may be based on old Honda blueprints, the quality is not even close either in material or workmanship.  In my experience, the Honda parts are not interchangeable.  The gaps and tolerances are not precise, bolts break and threads strip easily. I had to re-tap several threads due to poor metal quality.  The 70cc bike began burning oil at about 4 months and soon more smoke came from the engine case vent tube than from the exhaust pipe!  The kick starter on the 90cc broke as did the kick stand.  The nuts on all the bikes loosen easily and I felt it necessary to apply Loc-Tite to every nut and bolt I could find. 

The tires are terrible and wear very quickly.  The tubes puncture easily from sharp places on the spoke nipples.  In one week we went through 3 tubes on the 90cc before we realized/solved the problem.  We had a valve stem tear completely off the tube on the 2 stroke and couldn't find a replacement tube for 3 months for it's little 8" wheels.  I finally found a tube from a 12.5" toddler bicycle that worked. 

In fairness, the little 2 stroke has been the best, although the plastic fender broke off on the first crash because it was made of a brittle plastic.  It has a pull start which broke a couple of times but fortunately I was able to rewind the steel coil spring.  Parts are difficult to find and often costly.  For example, a replacement fender costs more than 10% of what the bike cost and the shock on the 70cc cost 10% of what the bike cost.  It doesn't take long to realize you've purchased a money pit. 

Plus, the paint might have lead in it, so don't gnaw on it!  Hopefully the Chinese manufacturers will drastically improve their quality but for now I will continue to be skeptical of these Chinese products.  Buyer beware.