These plugs have been a revelation/revolution for me.
Like many others here, I have struggled to get a decent signal from the wireless network in my house (it's built of brieze blocks, including all internal walls). Even my (very expensive) 'N' router is a waste of time.
I have a computer upstairs, at the other end of the house on which all files, documents, photos, music, video are stored. Communication with this has always been a struggle with wireless but I plugged these in - one to that machine and one to the router downstairs and OMG EVERYTHING IS PERFECT!!
The utility you get with the plugs shows I'm getting 85Mbps throughput AND I HAVE EXTENSION CABLES PLUGGED INTO EXTENSION CABLES. I really can't believe this, having read other peoples problems, but it is what I'm getting.
I now have video streaming to my TV and music going to my music server AT THE SAME TIME without glitches. File transfer is great; I transferred my entire MP3 collection (20GB)between machines completely reliably in minutes - well, quite a few minutes but minutes, nevertheless.
So why only three stars?
Well, these things are BIG and UGLY and produce a lot of heat - I don't feel comfortable leaving them plugged in, so it's an extra job when I leave the house. I wish they would go into a low power sleep mode when the network was not in use.
I'm also a bit concerned about the potential reliability issues identified by others and I will update this review if I have any problems.
But in basic performance terms, I have found these beyond criticism
UPDATE 09/05/2011: Still going strong - so no reliability issues!