Until someone comes up with a Thunderbolt-Gigabit-Ethernet Adapter, this one is the fastest, what you can get from USB 2. No, you won't get true Gigabit-Ethernet speed although the adapter advertises itself as a Gigabit-Ethernet port to your switch. The bottleneck is the USB port. I don't blame Plugeable for this. But they should make it clearer, that you won't get real Gigabit-Ethernet speed.
What you get is 300-400 Mbit/s. And that is still 3-4 times faster, than Apples USB-Ethernet adapter and 10 times faster, than the real WLAN throughput of your Mac Book Air. Works perfectly with Lion after installing the latest driver.
Would recommend this as a standard accessory for every Mac Book Air owner.
Pete