Just bought the new MacBook Air, 13" and the new AirPort Extreme Base Station. I am curious as to how I get the max connection? Option + Click on my WiFi shows I am connected only using "n" and on the 2.4Ghz frequency. Why would it not be on "ac" and 5Ghz?
Operating at 5GHz means that (in the margin - there are other factors) 802.11ac will penetrate walls, ceilings, floors less well than 802.11n at 2.4GHz.
The AirPort Utility allows you to setup a separate Network Name for the 5GHz frequency (Wireless Tab / Click Wireless Options). This means that you can explicitly select whether to connect a device via n/2.4GHz or ac/5GHz.
If you do this you can use /System/Library/CoreServices/Wireless Diagnostics (via Option+Click on WiFi) described here: How to troubleshoot 802.11ac bandwidth problems (note don't continue with the default assistant - select Utilities from the Window menu) to examine the relative performance of ac/n from different locations.
In my case I observed that with clear line of sight ac reported a Tx Rate of 867 Mbps compared to 145 Mbps for n. In contrast, several rooms away ac dropped off to the extent that n had better performance.
Reverting the changes above (so both 2.4GHz and 5GHz have the same network name) and repeating the tests, saw the connection automatically switch from ac to n as signal dropped off.
In contrast, I've not (yet) seen an automatic switch from n to ac at a location where powering Wifi off/on gets (and holds) the ac connection.
While less than perfect in terms of "just doing the right thing", this is perhaps understandable - triggering a switch when quality drops below some threshold rather than continuously looking for a "better" connection when what you've got (albeit n rather than ac) is rated as "excellent". Of course this is an OS X behaviour ... perhaps Mavericks will address it?
It seems to be a global problem affecting a lot of new MacBook Air. You may need to wait a software update.