This is a guide for anyone who is considering buying a Chinese motorbike or quad.
Chinese bikes and quads are a lot cheaper than their Japanese equivalents,but their build quality and finish is not nearly as good,and they often have design mistakes.If you enjoy adjusting/checking things,and making the electrics work properly,you are doing fairly low mileages,and you want the bike as a hobby,then go for it.This is a generalisation,but all Chinese bikes are not the same,and even one model (like the GY series) varies quite a lot between different sellers in terms of quality and parts,which are often not inter-changeable between different seller's bikes.If you buy from a well known large dealer,whose prices are higher than other people's,the bike will get a better check before you receive it,it'll be a bit better built,and the after-sales will be better,but the same fundamental problems will probably still arise.
Spare parts are generally cheap (not Thumpstars though,which are expensive) but this is largely negated by the need to replace them quite often,and more importantly by sellers' insistence on selling you complete large assemblies instead of  their component parts.For example,I rode my GY200 at Christmas when there was salt on the road,and the chrome on one of the forks became pitted.I could not buy the seals and the one fork tube;it was a complete new front forks assembly or nothing.
I have had one Chinese monkey bike,a Thumpstar (which are also Chinese), a GY200, and four Chinese quads.One of the quads is made by Lawnflite,and I think it's made in China,and it's the only one that could compete with a Japanese product,but the official RRP for a 100cc two-stroke quad is about £1100,which explains it really .It has had no problems at all.
I did the largest mileage,about 1,300,on the GY200,which was bought from the cheapest Ebay seller.From new,I needed to weld an extra piece of metal into the side stand,as it was too short,solder a broken electrical connection,and weld a piece into the rear rack to make it less weak.
After riding it,I noticed that the rear shock was the wrong rate,as it was left over from the pre-rising rate models,and would only achieve full compression with an elephant jumping over a double-decker bus.(this is a guess),so I bought a new one.This problem is now probably fixed on newer versions.
The rear brake failed completely at Xmas,and was fixed by tightening up the banjo bolts and re-bleeding it.Please see fork problems above.
Though they are cheap,they also depreciate quite fast,on mine,about 40% in a year,but they are probably better now as they are significantly cheaper new.
On the plus side,the bike was great fun,cheap,and quite fast,and before it started losing fork oil,handled OK.
If you get one,change the oil,and check the valve gaps and head torque before you drive it,and repeat after 500 miles.
To sum up,you get what you pay for,more or less.