I am using “Cable Matters® SuperSpeed USB 3.0 to RJ45 Gigabit Ethernet Network Adapter” plugged into USB 3.0 port of Lenovo U410 laptop (Windows 8.0)
When speed tested on speedtest.net, it only shows speed 85-95Mbit. In my office network is 1Gbit (tested, shows 700-800Mbit connection with other computers).
My laptop has 100Mbit card in it, so when I use native RJ45 port, speed is 100Mbit again.
Also, I tested it on MacBook Pro with Mac OS X 10.10 (Yosemite), and there speed also limited to 90Mbit.
I looked through google, but nobody seems to raise this issue. It is not issue with USB 2.0 port pretending being USB 3.0 because USB 2.0 should allow up to 500Mbit connection.
I wonder whether that is an issue with drivers (I installed from CD coming with unit), item itself, or just false advertisement.
UPDATE Cables and ports are important for connecting to ethernet. However, it seems that the device is either faulty or doesn't work at all by design. Also I couldn't find any success stories where people used it to get 1Gbps speeds. I couldn't on my Win8 or MacOS X machines.