First off, this item is not enough *by itself* to convert a coax BNC cable into an Ethernet cable: both the devices on both sides of the link have to support ePOE, which, as I understand it is proprietary to Dahua.
If you do have such devices (I used the DH-LR2110-8ET-120 switch and a few N43A* cameras), this an excellent way to upgrade an existing analog security camera system to IP without running new wiring. The connection is very stable, and plenty fast enough for 4mp at 30fps. A key advantage of using these passive adapters in combination with devices that natively support ePOE -- as opposed to using ePOE transceivers, which can be used to connect regular POE devices -- is that this adaptor is much more compact, allowing it to be squeezed into a regular-sized camera mounting junction box.