I recently purchased a Gigabit ethernet adapter for my rMBP. If you're fine with something a bit larger and are willing to go above "dirt cheap" I'd strongly recommend the Inateck HBU3VL3-4. It's made by a German company so I'm not sure how available it is over in the USA.

It has an ASIX AX88179 which is a USB 3.0 to Gigabit chip and it my tests I was easily able to get around 800-900Mbps. It also has 3x USB 3.0 ports, so not only do you not lose your USB 3.0 port you gain 2 more. I've not dismantled it but it feels solidly built.

Here in the UK I paid the same price as the Apple USB to Ethernet Adapter, which is only 100BASE-T and offers no hub.