I'm not seeing any interference issues, but that's because we tend to be either upstairs or downstairs, not both at once. This whole "take you turn" thing, if it really works, is probably is why it's working for us.

My router offers "up to 270Mbits/sec" or "up to 130Mbits/sec". Can I assume those map to 40 and 20MHz, respectively? I certainly don't want to be gratuitously unfriendly to my neighbors, but it's not like any of them are complaining.