BT use them wholesale for their BT Vision service (can you tell what I do
).
They use the Comtrend models not the Zyxel kit and when they're up and running they're fine. If they go wrong your generally screwed however.
By the way, I can't speak for the Zyxel kit but take a look at their power consumption. Comtrend models are astronomical. Certainly something I wouldn't use ever.
Again, if the Zyxel kit works in the same fashion as the Comtrend kit then you can have hundreds in the house. They have their own indepedant IP addressing so you could have around 255 around the house if needed. However, you'd probably be using about 10gigwatts of power a second if you did!
Oh and you can plug either end into a hub/switch/router etc. However, I have had the occasional problem if you run more than one homeplug/powerline adaptor off one hub/switch/router.