Not good for a house, but very useful for apartments in large buildings. You can have one wireless AP in each room (connected to eachother with wires), and almost no interference from the neighbours.
802.11n on the 5GHz band (most implementations use 2.4GHz) already does this. The real problem is the 2.4GHz band, which is simply over-saturated. 2.4GHz also penetrates walls really well, which is a drawback in apartment situations because it introduces more interference from neighboring access points (and many other devices operating on or near the 2.4GHz frequency - wireless phones, microwaves, etc.).
I've used a 5GHz 802.11n AP in my last two apartments; it's worked great every time. Most of the time I don't even see any other access points in range. Even if there are other APs, 5GHz has many more frequency bands available, so avoiding interference is simple. The only drawback is that so many devices don't support 5GHz. My smartphone didn't until I upgraded to the SGS III, and my Nook still doesn't.
Also, 5GHz is faster because the spec allows 40MHz channels.
Yes, but it can also use several channels. This plus the AC standard becoming more popular will lead to overcrowding in 5 GHz also.
2.4 GHz has 3 non-overlapping 20 MHz channels.
5 GHz has about 23 non-overlapping 20 MHz channels.
Both go through walls, so you will get interference from neighbouring apartments no matter what technology you implement. The only solutions are to use a much higher frequency, or shield your apartment walls.
