Linksys RE6500 wireless range extender review/help

Anyone here running a Linksys RE6500?

Just wondering how they hold up and if the speed drop is actually noticeable...

I'm currently stuck using a Netgear WN623, but the performance is garbage. I’m dropping from 17 Mbps down to like 3 or 4...

So what are we looking at for the wireless setup? Distance, walls, whatever else is getting in the way?

My modem is upstairs and the repeater is downstairs—not directly underneath, but off to the side—about 15 feet away, separated by a thick 12-inch concrete wall. I need a decent repeater to actually get a signal in the rest of the first floor. I'm using a Netgear WN623 right now, but the speed is garbage and totally unstable. It keeps bouncing between 0.5 and 7 Mbps during tests...