I use an AC66U but with 2 cisco AP in bridge 2.4Ghz.
I am planning to get another for the 5GHz network, i am waiting for this new one from asus with 6 antennas.
The speed is quite nice but as i am on linux (laptop) i can only test n speed.
They are not set optimal yet but will be it later, as i want to cover the outside area.
Maybe change channel too.