We have had our Wanhao Duplicator 5S for over a year, and with the exception of a few design flaws the machine has been an excellent addition.
The biggest problem we have had is with the bowden connector that keeps the plastic feeding tube connected to the filament feeder. The teeth inside of the bowden connector is prone to breaking and twice on long print jobs we've had this part break, ruining our prints (see picture below)
We have finally found a cheap and easy solution. We purchased some non-slip heat shrink wire wrap and attached it to the bowden connector. To ensure it stayed in place, we started the shrink wire wrap on the threading for the bowden connector so that part of it would be fastened down into the feeder with the threading of the nut. In addition, we attached a zip-tie around the plastic lip of the bowden connector to ensure it doesn't slip (see picture below).
In addition, we replaced the allen screw with a longer screw and added a clamp to ensure the plastic tube doesn't slip off. Since this modification, we haven't had any problems with the straw being pulled out of the bowden connector, even though our bowden connector no longer has teeth. Below you can see one of the failed prints due to the failure of the bowden connector.