Just bought a xy250gy-2a here in Qingdao, and I am off to explore the local mountains :-)

Does anyone have some documentation such as Wiring Diagram and User Manual?

Overall I'm pretty happy with it so far.
There are a few issues, but they are minor:
The turn signal is out of order. One of the signals is very dim. During troubleshooting I accidentally short circuited the blinker relay, so that also stopped working :-S (shit happens).
The rubber strap to hold the front fairing and headlight is missing a chunk of the hitch, but it still works fine.
The screw for holding had the thread partly stripped during the dealers installation. The problem is that the nut in the seat is not aligned properly with the guide in the frame. It is not a big issue now, but it could be a pain later.
The worst problem is maybe that some of the frame welding is porous. On mine it is at the point just in front of the gas tank where the top of the frame is welded to the head tube.
The last thing is that Neutral can be a bit hard to find.

The overall quality of the bike makes me confident that I will not have to walk home. The first bikes I looked at only cost 6000 RMB, but I think that money would be more or less wasted first time I would have a break down.

The bike rides well and has a decent acceleration.

Are there any fellow riders in Qingdao that would like to hook up?