I recently became aware of a USB 802.11ac adapter from ∑dimax for the Mac.
Having had good luck in the past with cheapie 802.11n adapters from ∑dimax on Linux platforms (notably the cheap ARM stuff that I have blogged about) I figured I'd give it a whirl.
Color me disappointed. The driver (yes, I upgraded to the most recent one from their web site) is a total amateur job. It is not integrated at all with the Mac's "Network" preference pane and is a separate icon in the menu bar. That might not be so bad if it actually kept track of the SSID association across reboots or turning WiFi off/on (it doesn't), or at least retained the password for my WPA2-Personal network (it doesn't), but the icing on the cake is that the performance is noticeably slower than the built-in 802.11n networking that came with the Mac.
Copying four video files up to a local NAS after getting them off the SSD and into memory with tar, I averaged 9.9MByte/sec throughput on the USB 802.11ac adaptor and 17.2 MByte/sec with the built-in 802.11n adaptor.
The Mac is a MacBookPro8,3 with 8GByte of RAM. The access point is a Ruckus R700, and we live on 10 acres in the country so there are no neighbors to interfere with us.
This one is getting returned. I'm surprised it has four stars on Amazon. Weird.