I heard some stories of poor castings on early tractors, but have not experienced any problems in almost 3 years of importing in either Jinma or KAMA products. Possible exception being a couple of broken bolts, but it is hard to tell what happended in those cases.
The reason to buy any tractor over another brand is value, usually perceived value. Some people find security in paying thousands extra for established brand with a dealer on the corner to take care of every glitch. others find value in having fewer dollars invested in a tractor, especially if they will only use it 100-150 hours per year or less. It is a very personal decision.
If you need/want things like Hydrostatic trans, cup holders, smooth shiney paint (getting better) than the China tractors are not for you (at this time).
At our dealership we sell KAMA a few Jinmas and Farm Pro as well as higher quality, more refined tractors from Kioti and Farmtrac. For most of my customers, the chinese tractor is all they need or want, but for others I encourage them to look at the more expensive, better finished brands.
The buyer of a Chinese tractor must be self reliant when it comes to repairs, most are sold with parts only warranty so all the labor is up to the buyer. These buyers feel the siginificant cost savings justifies this and in fact the majority of Chinese tractor owners are very satisfied with thier purchase.