I agree with Aleman.
Chinedolls are a GREAT introduction to motorizing bicycles. You will quickly learn about proper upkeep and like he said, if you ride her like she is meant to be ridden, you can have years of fun on the cheap.
95% of the issues I see are due entirely to the rider flogging her too hard or to poor upkeep. Generally, if a chine doll starts and runs for more than 30 minutes, it will last as long as the owner lets it last. I know of several chinadolls with over 10k miles. I sold one myself still running today. On a total POS Next crap frame no less. It's all about how you treat the motor.
Speed freaks seldom have much good to say of the chinadoll, but guys like me who just wanna ride cheap LOVE them. I will admit I'm prejudiced towards two strokes from years of open class MX racing, but these are really very well designed engines for the price. One of the simplest, most reliable motors ever built in my opinion. Given even half decent plating on the jug, there are only three moving parts: Piston, Crank, Rod.
You four stroke guys can talk all day about how much better they are for reliability but Very few four stroke designs would even start if built to the quality of a typical chinadoll
engine.
Given the proper love and care, I would pit a chinadoll against anything ever built for cheapest way to get from A to B.