I'm also curious about these devices. Right now I'm using wifi to connect my HTPC to my ADSL router (upstairs) which is sufficient for browsing but streaming HD content and transferring files is a big no-no.
I was therefore thinking of using Homeplug ethernet adapters as a possible solution however I've read that the performance of these things depend heavily on the layout of the electrical wiring in the house as well as the phases etc. I don't want to purchase something like this only to find out it doesn't work very well.
I haven't found them to work very well, it's very dependent on the way your house is wired. If speed is important then nothing beats good, old-fashioned cable. Otherwise start with a wireless-n router, and get an external antenna if needed.