Repeater mode will not cut the speed in half for devices that are connected to the repeater's LAN ports. You will see a drop though in devices that connect via the repeater's wireless SSID's, with a greater drop on 2.4ghz than on 5ghz.
I am currently using two AC66U's, one as a router and the other as repeater. The repeater is set to use the 5ghz channel from the router which it receives and then rebroadcasts on both the repeater's 2.4 and 5ghz radios, as well distributing via the Gigabit Ethernet LAN ports to devices that I connect via LAN. The two devices sit about 18 feet away from each other, at an angle (since they are on different floors, with the router upstairs and the repeater downstairs). I have an HTPC running Windows 7 that I use VLC to stream high bit-rate ISO videos and bluerays, which is connected directly via one of the LAN ports on the repeater. The connection rate is at 725Mbps (so it's close to the theoretical max). I've measured file transfer speeds, and I can get close to 60MB/s, and with internet download speeds, I'm on a 300/20 plan from TimeWarnerCable, and I can routinely test at 320 down and 21 up every time on this computer. So, it's clear that speed isn't "cut in half" for LAN wired connections and that in this regard, the Repeater is essentially functioning as a wireless bridge for wired clients connected to it.
Speeds are cut, but not by half with wifi, by about 1/3, but of course, this is also affected by distance of the wireless client that is connecting. The highest speeds are still on 5ghz (as well as the highest throughput). Both 2.4 and 5ghz are solid.
Of course, running a cable to the second unit is preferable (i.e., using it as a true Access Point), but the Repeater function is pretty nice on Asus routers. I can recommend it highly.
