What makes a China product ( or probably taiwanese) mt. bike a good buy ? I mean, are they not bad at all?

There are very good quality mountain bikes built both in China and Taiwan. There are also very poor quality bikes made in both places. However, there is no magic formula whereby a company van produce a high quality product at a very low price. The better bikes will simply cost more to make and to buy. A good bike will sell for $500 or more

Cheap bikes are found at Target, Kmart, Sears & the dreaded Wally World. Good bikes are found at REAL bicycle shops. You want a "good" MTB? Plan on at least $550 - $600, if not more depending on where you plan on riding. A good full suspension bike will set you back thousands.

Cheep crapy metirials and poor craftsmanship