Both channels work at N rates for me, and I've confirmed it with two different laptops. XP tells me that my speed on the N based interfaces says 270M. My son uses his N on 2.4 for SW Development College coursework. He much prefers it to the wired 100M, says that it is decently faster. (GB router linked to 100MB switch he connects to. Wireless N, of course is direct.)
I have a Linksys E3000 and use a WUSB600Nv2 dongle on my HP XP pro laptop. Have had connections at N rates on 802.11A frequency. Seems to work all right, but this was for testing and stats only.
I have not tried media serving at all, to a media device, since I don't got none…
I have had the best luck with Toastman's build and have been using the Tomato Firmware v1.28.7440 MIPSR2-Toastman E3000 USB Ext build since mid-late January with no real issues.