I'm not sure I understand the benefit / speed difference between an AC router (or even N) and a "wired" Ethernet connection??

The big advantage to Wireless-AC vs. Wireless-N is when you're accessing internal media. For example, if you have a laptop that is running a backup to your connected hard drive on the USB port and you're also streaming a movie in 4k off of your local media server at the same time on your TV. Wireless-AC then has a HUGE advantage over Wireless-N. Your max available bandwidth with this router would be 1300 mbps on the AC (5G) band. If both of your devices are using the AC band, they may each only connect at 450 mbps for a total of 900mbps of bandwidth (out of a possible 1300mbps). So the router could accommodate this type of activity from your 2 devices. If you tried this using the Wireless-N band, your results would not be as smooth when you have 2 devices performing these same types of activity. I've seen Wireless-N connections drop down as far as 72 mbps if the signal is anything less than excellent. If you're streaming 4k video and running a backup at these speeds, the performance will be much less than ideal. I always connect my TVs to the 5G network whenever possible for this reason.

When in Access Point mode, are the guest networks firewalled off from your private network?

Yes they can be blocked from accessing your private network. I do it all of the time on my RT-N66U and I just downloaded the manual for the RT-AC66U and it has the same settings. To block guests from accessing your private network, go to "guest Network" and click enable. You will then see the config information for the guest network. All that needs to be done is choose Disable for "Access Intranet", which I think is the default. I usually set my access time for 3 hours on an open network which allows guests to easily connect and then after 3 hours it automatically turns off the guest network. Couldn't be simpler. Only issue that I have is when I enable the guest network it drops all wireless connections for a second or two.

does this support dd-wrt?

I was pretty happy with Merlin as well; the current version of the firmware as of July 1 2016 has bugs that prevent repeater mode from working, but Merlin worked fine. Do *not* upgrade to the latest version of the ASUS firmware, since it has the "FCC fixes" that prevent you from loading DD-WRT or Merlin.

Standing screen display size	8.1 Inches
Wireless Type	802.11ac
Number of USB 2.0 Ports	1
Number of USB 3.0 Ports	1
Brand	ASUS
Series	RT-AC66U
Item model number	RT-AC66U (AC1750)
Operating System	Windows 7, Mac OS
Item Weight	1.46 pounds
Product Dimensions	8.14 x 5.86 x 1.4 inches
Item Dimensions LxWxH	8.14 x 5.86 x 1.4 inches
Color	Black
Computer Memory Type	DRAM
Voltage	19 Volts
Batteries	1 AAA batteries required.
Manufacturer	ASUS
Language	English
ASIN	B008ABOJKS
Is Discontinued By Manufacturer	No
Date First Available	June 10, 2012

ASUS Dual-band 3x3 AC1750 Wifi 4-port Gigabit Router with speeds up to 1750Mbps & AiRadar to strengthens Wireless Connections via High-powered Amplification Beam-forming - 2x USB 2.0 Ports (RT-AC66U)

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