Despite the number of poor reviews I decided to risk trying one of these. Mine came from NCWholesale. I've no idea if all of the sellers offering this have an identical product or even if individual sellers are actually getting the same product from batch to batch as substituting parts isn't unusual in this sort of low cost generic device.
The device I received is definitely a full speed device (i.e. 12 Mbps not 480 Mbps / high speed) despite the description string in the USB descriptor saying "USB 2.0 10/100M Ethernet Adaptor". The rest of the USB descriptor identifies it as USB 1.10, full speed, Vendor 0fe6, ProdID 9700, MaxPower 120 mA which leads to the conclusion it is using a Davicom DM9601 chip. As I only want it to carry traffic off a broadband connection that never gets close to 12 Mbps then full speed is quite fast enough but it won't be able to make full use of a 100 Mbps Ethernet. The 120 mA power requirement also means it won't work in some scenarios, for example unpowered hubs only guarantee to be able to provide 100 mA per port.
If you are using it under Linux be aware that the minimum kernel version that includes a compatible driver is 2.6.38.There is a linux driver on the supplied CD but the accompanying compilation instructions are in chinese. There is also an android driver which appears to be the same code except it has had the word android scattered through it almost at random. WinXP, Vista Win7 and Mac drivers are also included but I didn't look at them in any depth.
Having got a suitable driver loaded it does appear to work.