I have taken the plunge and very recently bought the RT-AC66U.
I am using the router in my 990 square-foot apartment and the performance in the living room (where is router is) is great - up to 90Mbps (my fibre link is 100Mbps) using speed test on my iPhone 5 and Samsung Galaxy s3 as well as iPad 4.. The router is placed 0.8 meter above the ground and I'm running 802.11n devices.
However, down the 4 meter narrow corridor into my bedroom the performance dropped to pretty bad, with signal strength showing as decent but the speed is very slow - less than 1Mbps. Sometime I cannot even join the network. Distance is maximum of 15 meter from the router.
When I tried this in the bathroom in my bedroom (just a little deeper in), I get nothing.
I tried changing through all the control channel with some maybe having slightly better performance at times but it is very unstable and uncertain. In the bathroom, still nothing.
Increasing tx power didn't help. Positioning antenna in different ways didn't help and elevating it didn't make much difference.
My place is only 990 sq foot whereas tests of this router have been done in much larger houses.
This is the same issue as my cheap Huawei and Dlink router and I thought the RT-AC66U would surely overcome this obstacle. I am still hopeful - maybe some other settings?
Would greatly appreciate it if you can advise how I get get the wifi range to reach through that narrow corridor into the bedroom.
Using that router as just an N router you will not get a signal that is better from an N router (although it is a good N router), the quality that you receive at any spot depends on local interference from neighbors and the quality and exact type of the N wireless adapter used. I would check out the other signals nearby using the free inSSIDer analyzer. It is very early to go AC and there will be firmware updates in the future that may help.
Are you using it on N only with WPA2/AES security? You cannot allow any legacy G devices to connect or it will slow and shorten your range.
The best way to extend the signal is by adding a wireless access point connected by Ethernet cable to the router. You can use one of your old routers configured as an AP.